Also adds more trace events to GPU backend. Change-Id: Ifa5f0cd4b1fd582f0cc30d37d9e6414dc498c75d Reviewed-on: https://skia-review.googlesource.com/24622 Reviewed-by: Greg Daniel <egdaniel@google.com> Commit-Queue: Brian Salomon <bsalomon@google.com>
