Support jumbo in the //gpu code

Jumbo is a unity build system for Chromium. By compiling many
cc files as one unit, the compile time is drastically reduced. This
patch adds support for jumbo compiling //gpu.

The expected effect is about 6 CPU minutes (~1 wall clock minute on a
normal computer) or 1% of the total reference jumbo compilation time.

Bug: 864986
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I096335ba283f9b03cf4dcde554e7b19620f08834
Reviewed-on: https://chromium-review.googlesource.com/1151323
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#590626}
7 files changed