Move WebGraphicsContext3D::Attributes to Platform::ContextAttributes.
Also moves the url to be a parameter of the create method instead of
being an attribute, which lets us pass it by reference, which lets
us use a WebURL instead of doing conversions to/from WebStrings.
R=chrishtr@chromium.org,
kbr@chromium.org, piman, pfeldman
TBR=pfeldman
BUG=
584497
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel
Committed:
https://crrev.com/6ee20a04d40a4fa2ca9930325fa3173a0247817a
Cr-Commit-Position: refs/heads/master@{#384766}