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Jumbo isn't compatible with "using namespace" since those cause
compiler warnings unless in the top level file, but "using namespace"
is also prohibited by the code style guide (see:
https://google.github.io/styleguide/cppguide.html#Namespaces )

This makes the code a bit more verbose, but it also makes it
more self consistent since the gles2:: prefix was already
occasionally used.

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: I7c2111e70407a7ddf65473cb83e7f63b70b18747
Reviewed-on: https://chromium-review.googlesource.com/1141882Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#576159}