I came across these three classes in the last CL, so do some
cleanup as I touched these classes.

BUG=
TEST=webgl conformance, no regressions

Change-Id: I2819bab5965fcb4917f85c2eded571a7f95ab9a2
Reviewed-on: https://chromium-review.googlesource.com/199423Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>