Webkit: Change old GPU Header path to new location

The path external/skia/gpu/* is no longer used in JB.
It has changed to external/skia/src/gpu/
This patch set has been submitted to Google as well:
https://android-review.googlesource.com/#/c/41915/

Change-Id: Ib2b1fad1a2cbd53bf91beb7c2e2ac44cc0ae470b