Add Goma support in gn_flavor.

Using full paths in skia.h causes "file not found" with Goma. All other
Builds seem fine without the path, so I changed find_headers.py to use
the basename.

Change-Id: Ib520e91a92ebffe36a736eb53f643d359f5bb2ce
Reviewed-on: https://skia-review.googlesource.com/79360
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
7 files changed