Use Platform::ContextType as the webgl version type

This patch uses Platform::ContextType instead of unsigned int as
the webgl version type. Meanwhile, a new context type
kWebGL2ComputeContextType is added to correspond to the
webgl2-compute context.

Bug: 859249
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_layout_tests_slimming_paint_v2;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I6b8a19970c33e3a35ba54c38a0e55acccda567e3
Reviewed-on: https://chromium-review.googlesource.com/1136224
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Cr-Commit-Position: refs/heads/master@{#575551}
22 files changed