Cache the tracer category flag for graphics_context_annotations.

We're currently calling EventTracer::getTraceCategoryEnabledFlag() on
every paintContents(), and the category lookup has a significant
overhead (linear string search). We should be caching and reusing the
result instead.

BUG=259433
R=nduca@chromium.org

Review URL: https://codereview.chromium.org/54753002

git-svn-id: svn://svn.chromium.org/blink/trunk@161102 bbb929c8-8fbe-4397-9dbb-9b2b20218538