GYP: Merge svg_idl_files list into core_idl_files

Minor simplification: there is no need for two separate lists:
SVG files handled the same for the build.

Review URL: https://codereview.chromium.org/135653004

git-svn-id: svn://svn.chromium.org/blink/trunk@165197 bbb929c8-8fbe-4397-9dbb-9b2b20218538