Fix build summary to classify *.mojom*.obj as .obj

The build summary script was overly aggressive about summarizing .mojom
files under the mojo category. This was intended to measure the cost of
generating source from mojo files but it accidentally pulled in the
.mojom object files.

Categorizing different types of object files might be worthwhile but
that should be done intentionally and consistently.

Change-Id: Iab6b7e94797ce7f1ed46805034b4f274c88617e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2067647
Commit-Queue: Aaron Gable <agable@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>

post_build_ninja_summary.py

@@ -170,10 +170,6 @@ def GetExtension(target):
   consistent 'canonical' extension. Ultimately the goal is to group build steps
   by type."""
   for output in target.targets:
-    # Normalize all mojo related outputs to 'mojo'.
-    if output.count('.mojom') > 0:
-      extension = 'mojo'
-      break
     # Not a true extension, but a good grouping.
     if output.endswith('type_mappings'):
       extension = 'type_mappings'
@@ -190,6 +186,13 @@ def GetExtension(target):
       extension = '.so (linking)'
       # Attempt to identify linking, avoid identifying as '.TOC'
       break
+    # Make sure .obj files don't get categorized as mojo files
+    if extension in ['.obj', '.o']:
+      break
+    # Normalize all mojo related outputs to 'mojo'.
+    if output.count('.mojom') > 0:
+      extension = 'mojo'
+      break
   return extension