If our gyp does not exist, use chromium's.

If our gyp does not exist, use chromium's.

The previous version had faith that if the third_party folder
existed, it must contain externals/gyp. A recent change added
third_party (in git, as opposed to using DEPS), so now Android
has third_party but not externals/gyp.

Now check the full filepath. If it's not there, back up to
chromium's version.

Fixes Android canary bot.

R=halcanary@google.com
TBR=halcanary@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/295103008

git-svn-id: http://skia.googlecode.com/svn/trunk@14855 2bbb7eff-a529-9590-31e7-b0007b416f81