A few different tests are blacklisting the WebGL GPU feature. This will
no longer work when SwiftShader is fully integrated in chromium, as it
will become a fallback for GPU accelerated WebGL, and the WebGL feature
will stay enabled.

In order to fix this issue, the WebGL feature will now only be enabled
when accelerated. When SwiftShader is used, these features will now be
disabled. A new utility function, IsWebGLEnabled(), which checks for
both swiftshader and the WebGL feature, was added in order to verify if
WebGL can actually be used by chromium.

BUG=630728
R=rdevlin.cronin@chromium.org, jbauman@chromium.org, zmo@chromium.org, piman@chromium.org
TBR=laforge@chromium.org
TBR_REASON=Trivial change to swiftshader_component_installer.cc, which will soon be deleted entirely
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2737983002
Cr-Commit-Position: refs/heads/master@{#456405}