Implement tex{Sub}Image3D taking sub-rectangles of HTMLImageElements.

Tested with new tests in
https://github.com/KhronosGroup/WebGL/pull/2132 .

Also includes the following needed WebGL conformance roll:

Roll WebGL 7516ab7..bc2b06d

https://chromium.googlesource.com/external/khronosgroup/webgl.git/+log/7516ab7..bc2b06d

BUG=639145
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2482793005
Cr-Commit-Position: refs/heads/master@{#430762}