Add onMemoryDump to GrContext

Adds an entry point to GrContext to allow enumeration and tracing of GPU resources
via the newly added SkTraceMemoryDump.

Plan is for Chrome to call this on each of its GrContexts.

Dumps both the total size of GPU resources, as well as the total purgeable size.

BUG=526261

Review URL: https://codereview.chromium.org/1313743002
20 files changed