GitLab

Remove ui::ContextProvider and ui::GLES2Context, since they are incomplete.
Instead use ui::ContextProviderCommandBuffer, which is used by chrome browser
and renderers.

BUG=643746
TBR=jam@ for content/renderer for code update for API change

Review-Url: https://codereview.chromium.org/2586323002
Cr-Commit-Position: refs/heads/master@{#439718}