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It isn't necessary to mess around with sharing CGLContextObjs anymore,
they were necessary only to share data between the frame subscriber
and the RenderWidgetHostViewMac. Just give each IOSurfaceLayer its
own CGLContextObj, and draw using immediate mode (using shaders
resulted in an increase in startup time).

BUG=314190
TBR=pfeldman@chromium.org
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/490393002

Cr-Commit-Position: refs/heads/master@{#293103}