Change FileArg syntax
This changes the syntax from:
@(filepath:key0:key1)
to:
@FileArg(filepath:key0:key1)
This should make it much easier for someone who encounters this syntax
to figure out what it is (currently a `git grep FileArg` returns only
results about/using this feature).
Also changes it to use a simple regex search instead of adhoc stuff.
BUG=359249
Review URL: https://codereview.chromium.org/
377803003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283484 0039d316-1c4b-4281-b951-d872f2087c98