Add bullet proofing to image_subset GM for TSAN failures

The most common TSAN failure is:

	glmsaa8 gm  image_subset

with this stack trace:

5.73m	    /b/s/w/ir/build/dm(_ZNK7SkImage10makeSubsetERK7SkIRect+0x96) [0x1a7c356]
    /b/s/w/ir/build/dm(_ZNK7SkImage10makeSubsetERK7SkIRect+0x96) [0x1a7c356]
5.73m	    /b/s/w/ir/build/dm() [0x12cf587]
    /b/s/w/ir/build/dm() [0x12cf587]
5.73m	    /b/s/w/ir/build/dm(_ZN6skiagm8SimpleGM6onDrawEP8SkCanvas+0x25) [0x12ba205]

Change-Id: Ife1c27af935ef3f56073360e9781a89fe8206586
Reviewed-on: https://skia-review.googlesource.com/141280
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
2 files changed