On some Mac drivers with shader version 4.1, they will
treat unused std140 and shared uniform blocks' members as inactive. However,
WebGL2.0 based on OpenGL ES3.0.4 requires all members of a named uniform block
declared with a shared or std140 layout qualifier to be considered active.
The uniform block itself is also considered active.

This workaround is to reference all members of unused std140 and shared uniform blocks
at the beginning of the vertex/fragment shader's main().

BUG=chromium:618464
TEST=UniformBufferTest.ActiveUniformBlockNumber

Change-Id: I18da4e2b61b0170068bf5ea38ce54667b0737780
Reviewed-on: https://chromium-review.googlesource.com/395648Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>