[autoninja] Update Siso .ninja_log comments

Siso is going to generate non-empty .ninja_log for the analysis tools.
https://crrev.com/c/4907677

This CL fixes the comment about empty ninja_log.

It's not compatible with Ninja's .ninja_log. So `gn clean` will still
be required.

Bug: b/298594790
Change-Id: Ib4c60f3ed22f516d6f7e2847aaf57e228121eccf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4937692
Auto-Submit: Junji Watanabe <jwata@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>

autoninja.py

@@ -166,11 +166,11 @@ def main(args):
 
         siso_marker = os.path.join(output_dir, '.siso_deps')
         if use_siso:
-            ninja_marker = os.path.join(output_dir, '.ninja_log')
             # autosiso generates a .ninja_log file so the mere existence of a
             # .ninja_log file doesn't imply that a ninja build was done. However
             # if there is a .ninja_log but no .siso_deps then that implies a
             # ninja build.
+            ninja_marker = os.path.join(output_dir, '.ninja_log')
             if os.path.exists(ninja_marker) and not os.path.exists(siso_marker):
                 print('Run gn clean before switching from ninja to siso in %s' %
                       output_dir,

post_build_ninja_summary.py

@@ -117,8 +117,8 @@ def ReadTargets(log, show_all):
 
     The result is a list of Target objects."""
     header = log.readline()
-    # TODO: b/298594790 - Siso currently generates empty .ninja_log files.
-    # Handle them gracefully by silently returning an empty list of targets.
+    # Handle empty ninja_log gracefully by silently returning an empty list of
+    # targets.
     if not header:
         return []
     assert header == '# ninja log v5\n', \