Enable SwiftShader fallback on desktop Linux.

SwiftShader is an OpenGL ES implementation which runs on the CPU,
and is used as a fallback for WebGL support when the GPU or its driver
have been blacklisted. It has been added to the Chrome installer for
Windows in https://codereview.chromium.org/2715563002/ This CL enables
the integration in Chromium on Linux.

BUG=160392
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/2777193002
Cr-Commit-Position: refs/heads/master@{#463872}
7 files changed