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Currently, users of Vulkan need to get the global function pointer
class. And calls into those functions look like:
vulkan_function_pointers->vkCreateDevice();

Our OpenGL code hides that behind macros. This patch adds macros to
mimic the OpenGL code in Chrome.

Now that looks like:
vkCreateDevice();

BUG=856360,848101

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: If3aff1719b366748b9236a9e4c9e7ec69260be18
Reviewed-on: https://chromium-review.googlesource.com/1116195
Commit-Queue: Chris Blume <cblume@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571226}