Fix enum vs unsigned -Wsign-compare warnings

Recent Clang versions started taking into account that enums are signed
on Windows when emitting these warnings.

NOTRY=true

Bug: 792519
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I0fe2646985282b759e3e261a09fa0f1380c98e61
Reviewed-on: https://chromium-review.googlesource.com/813182
Commit-Queue: Hans Wennborg <[email protected]>
Reviewed-by: Jochen Eisinger <[email protected]>
Cr-Commit-Position: refs/heads/master@{#522454}
36 files changed