It seems reasonable to validate the equality between stencilWriteMask
and stencilBackWriteMask only by comparing the effective bits.
An existing dEQP test[1] and a WebGL test[2] also use the max size of
stencil bits for checking the stencil mask values.

[1] StencilWriteMaskSeparateTestCase (gles3/functional/
es3fIntegerStateQueryTests.cpp)
[2] https://www.khronos.org/registry/webgl/sdk/tests/conformance/state/
gl-get-calls.html (Search for "minimumRequiredStencilMask ")

Re-land with fix for Clang warning.

Change-Id: I4cadaffced04ca55fa6daf7ac6a462368e0cfeec
Reviewed-on: https://chromium-review.googlesource.com/307530Reviewed-by: Jinyoung Hur <hurims@gmail.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>