Load the GL_KHR_debug entry points and log messages they produce.

BUG=661603
CQ_INCLUDE_TRYBOTS=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/2472703003
Cr-Original-Commit-Position: refs/heads/master@{#429589}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 326c851cbb14cc379fa6516ef2cf73f18f8778f0