Return a size of 0 for external format GrSurfaces.

This fixes a case where Chrome was hitting the assert in ComputeSize that
colorSize > 0. In general this shouldn't affect our memory/budget tracking
since external textures are generally given to us as wrapped borrowed textures
so they're memory was not counted against our budget to begin with.

Change-Id: I2fd4eaaab52d9c3dc1af3a8f5b59a1b1f3c5dbb7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/256656
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed