Add a temporary variable to store ANGLE gyp file.

This will allow us to correctly transition to the new angle.gypi
from the old build_angle.gypi.

BUG=415893

Review URL: https://codereview.chromium.org/604413002

Cr-Commit-Position: refs/heads/master@{#297017}

build/common.gypi

@@ -2339,6 +2339,12 @@
     # The path to the ANGLE library.
     'angle_path': '<(DEPTH)/third_party/angle',
 
+    # TODO(jmadill): remove angle_gyp. http://crbug.com/415983
+    # This temporary variable lets us change the name of the gyp file
+    # in blink and in chromium simultaneously. It should go away after
+    # we're done changing the path.
+    'angle_gyp': 'build_angle.gyp',
+
     # List of default apps to install in new profiles.  The first list contains
     # the source files as found in svn.  The second list, used only for linux,
     # contains the destination location for each of the files.  When a crx