Avoid "using namespace gles2" in the gpu code

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/1141882
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#576159}
4 files changed