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When in the component build multiple threads (apps) will use the same
view manager client lib and could fight over the current GL context, so
always make our context current in BitmapUploader.

When shutting down abrubtly (i.e. when losing a connection to an app),
the client will not necessarily properly unref all resources it uses.
DCHECKing here isn't helpful, so just skip over resources that are not
fully unreffed.

R=ben@chromium.org

Review URL: https://codereview.chromium.org/569173007

Cr-Commit-Position: refs/heads/master@{#294943}