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The swapchain function pointer block is currently missing a header
comment.

This CL adds it.

BUG=856360

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: Iade801831fef5a4502f2c66852182f22ef4619aa
Reviewed-on: https://chromium-review.googlesource.com/1114220Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570221}