Added extension macro checks for android plugin files.
Some extension files are compiled on android when enable_plugins is set
to 1. These files need to have the parts that references extension
files compiled out of the code.
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3032
TEST= trybots
Review URL: https://codereview.chromium.org/
468913006
Cr-Commit-Position: refs/heads/master@{#291219}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291219 0039d316-1c4b-4281-b951-d872f2087c98