Speculatively clobber Mac after "Switch Mac Intel GPU bots to 10.12 from 10.10"

Something in this range broke the Mac builder:
https://chromium.googlesource.com/chromium/src/+log/c6e4c6f1f9865b5a77d2cc4eb8c7104b632624f1%5E..1a7740c44a6c4d03423c4b5ff7d746e1a1b7363e?pretty=fuller

r442828 is the most obvious candidate which may have affected builds.

BUG=679607
TBR=kbr@chromium.org

Review-Url: https://codereview.chromium.org/2626993002 .
Cr-Commit-Position: refs/heads/master@{#442832}

build/get_landmines.py

@@ -70,6 +70,8 @@ def print_landmines():
     print 'Clobber to get rid of evil libsqlite3.dylib (crbug.com/526208)'
   if platform() == 'mac':
     print 'Clobber to remove libsystem.dylib. See crbug.com/620075'
+  if platform() == 'mac':
+    print 'Clobber to get past mojo gen build error (crbug.com/679607)'
 
 
 def main():