Add a stale .pyc cleanup step before running gyp

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 8b63893f643c5247ce5a822394242bbf0d9120af..768a811af604969d78e305ca75af9d3bca3484f7 100644
--- a/DEPS
+++ b/DEPS
@@ -747,6 +747,22 @@ hooks = [
     'action': ['python', 'src/third_party/instrumented_libraries/scripts/download_binaries.py'],
   },
   {
+    # Ensure that while generating dependencies lists in .gyp files we don't accidentally

[scottmg, 2015/06/12 22:58:25]

nit; Wrap at 80 col since there's nothing preventing it here.

[mnaganov (inactive), 2015/06/12 23:26:17]

Done.

+    # reference any .pyc files whose corresponding .py files have already been deleted.
+    'name': 'remove_stale_pyc_files_for_gyp',

[scottmg, 2015/06/12 22:58:25]

Probably just 'remove_stale_pyc_files' is better.

[mnaganov (inactive), 2015/06/12 23:26:17]

Done.

+    'pattern': '.',
+    'action': [
+        'python',
+        'src/tools/remove_stale_pyc_files.py',
+        'src/android_webview/tools',
+        'src/gpu/gles2_conform_support',
+        'src/ppapi',
+        'src/printing',
+        'src/third_party/closure_compiler/build',
+        'src/tools',
+    ],

[scottmg, 2015/06/12 22:58:25]

I was worried this would be kind of slow, but on Windows it's not that bad:

tim cmd /c python tools\remove_stale_pyc_files.py android_webview/tools
gpu/gles2_conform_support ppapi printing third_party/closure_compiler/build
tools
real: 0m0.453s

(Doing '.' on the other hand takes 18s, so that's no good.)

[mnaganov (inactive), 2015/06/12 23:26:17]

RemoveAllStalePycFiles doesn't skip output dirs, and third_party dirs can also
be quite big (e.g. layout test results). It may be possible to parallelize this
task, but that's not easy due to recursive nature of the traversal.

+  },
+  {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     'name': 'gyp',
     'pattern': '.',