Pass large -j value to ninja when use_rbe = true

The goma-on-borg backend is turning down. While chromium can
continue using the goma client for build acceleration, the
backend is swapped out to instead use Remote Build Execution,
a gcloud service.

Other downstream projects (e.g. Nest) switched out the goma
frontend as well as the backend to RBE. These projects use a
|use_rbe| gn flag to enable build acceleration through RBE.

This change allows autoninja to recognize this flag and treat it
as if it was using goma for the purposes of passing an
appropriate -j flag.

Bug: None
Test: run on an out/ directory with use_rbe enabled

Change-Id: I52d5771e92e9163234811b154fd36816639bf1b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2391009
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Simeon Anfinrud <sanfin@chromium.org>

autoninja.py

@@ -5,10 +5,10 @@
 
 """
 This script (intended to be invoked by autoninja or autoninja.bat) detects
-whether a build is using goma. If so it runs with a large -j value, and
-otherwise it chooses a small one. This auto-adjustment makes using goma simpler
-and safer, and avoids errors that can cause slow goma builds or swap-storms
-on non-goma builds.
+whether a build is accelerated using a service like goma. If so, it runs with a
+large -j value, and otherwise it chooses a small one. This auto-adjustment
+makes using remote build acceleration simpler and safer, and avoids errors that
+can cause slow goma builds or swap-storms on unaccelerated builds.
 """
 
 # [VPYTHON:BEGIN]
@@ -57,23 +57,23 @@ for index, arg in enumerate(input_args[1:]):
     # Support -Cout/Default
     output_dir = arg[2:]
 
-use_goma = False
+use_remote_build = False
 
-# Attempt to auto-detect goma usage.  We support gn-based builds, where we
-# look for args.gn in the build tree, and cmake-based builds where we look for
-# rules.ninja.
+# Attempt to auto-detect remote build acceleration. We support gn-based
+# builds, where we look for args.gn in the build tree, and cmake-based builds
+# where we look for rules.ninja.
 if os.path.exists(os.path.join(output_dir, 'args.gn')):
   with open(os.path.join(output_dir, 'args.gn')) as file_handle:
     for line in file_handle:
-      # This regex pattern copied from create_installer_archive.py
-      if re.match(r'^\s*use_goma\s*=\s*true(\s*$|\s*#.*$)', line):
-        use_goma = True
+      # Either use_goma or use_rbe activate build acceleration.
+      if re.match(r'^\s*(use_goma|use_rbe)\s*=\s*true(\s*$|\s*#.*$)', line):
+        use_remote_build = True
         continue
 elif os.path.exists(os.path.join(output_dir, 'rules.ninja')):
   with open(os.path.join(output_dir, 'rules.ninja')) as file_handle:
     for line in file_handle:
       if re.match(r'^\s*command\s*=\s*\S+gomacc', line):
-        use_goma = True
+        use_remote_build = True
         break
 
 # If GOMA_DISABLED is set to "true", "t", "yes", "y", or "1" (case-insensitive)
@@ -85,7 +85,7 @@ elif os.path.exists(os.path.join(output_dir, 'rules.ninja')):
 # autoninja uses an appropriate -j value in this situation.
 goma_disabled_env = os.environ.get('GOMA_DISABLED', '0').lower()
 if goma_disabled_env in ['true', 't', 'yes', 'y', '1']:
-  use_goma = False
+  use_remote_build = False
 
 # Specify ninja.exe on Windows so that ninja.bat can call autoninja and not
 # be called back.
@@ -98,7 +98,7 @@ args = [ninja_exe_path] + input_args[1:]
 
 num_cores = psutil.cpu_count()
 if not j_specified and not t_specified:
-  if use_goma:
+  if use_remote_build:
     args.append('-j')
     core_multiplier = int(os.environ.get('NINJA_CORE_MULTIPLIER', '40'))
     j_value = num_cores * core_multiplier