Prevent garbage collection from deleting a context during its eviction.

BUG=374086
TEST=gpu linux debug bot on webkit waterfall, no random crashes
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/363443002

git-svn-id: svn://svn.chromium.org/blink/trunk@177435 bbb929c8-8fbe-4397-9dbb-9b2b20218538