Fix a GCC warning found by Skia.
Fix a GCC warning found by Skia.

This only seems to affect GCC's enum-compare warning,
because we were comparing an untyped enum with a size_t enum.

BUG=None 

Change-Id: I74b8315cff61344d1b7700b1c56a40d2ffce41e2
Reviewed-on: https://chromium-review.googlesource.com/698296
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

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