Add per-file rules for Source/web/WebBindings.cpp

Add per-file rules for Source/web/WebBindings.cpp so that bindings OWNERS
can review changes to that file.

R=abarth
BUG=

Review URL: https://chromiumcodereview.appspot.com/24036003

git-svn-id: svn://svn.chromium.org/blink/trunk@157883 bbb929c8-8fbe-4397-9dbb-9b2b20218538