Detect RBE builds and accelerate them.

Chromecast builds use a buildflag called `use_rbe` instead of
`use_remoteexec`.

Bug: None
Test: run `autoninja` in the chromecast internal repo
Change-Id: Iebb18ad47a30c4152923f48175222ec6a20e16f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4144469
Reviewed-by: Gavin Mak <gavinmak@google.com>
Auto-Submit: Simeon Anfinrud <sanfin@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>

autoninja.py

@@ -98,6 +98,9 @@ def main(args):
                      line_without_comment):
           use_remoteexec = True
           continue
+        if re.search(r'(^|\s)(use_rbe)\s*=\s*true($|\s)', line_without_comment):
+          use_remoteexec = True
+          continue
   else:
     for relative_path in [
         '',  # GN keeps them in the root of output_dir