Restructure flushing relationship between GrContext, GrDrawingManager, and GrResource...

Restructure flushing relationship between GrContext, GrDrawingManager, and GrResourceCache.

Consolidates all flush actions into GrDrawingManager and makes GrContext::flush a passthrough.

Removes the unused and untested discard flush variation.

Replaces the indirect overbudget callback mechanism of GrResourceCache with a flag set by resource cache when it wants to flush that is checked after each draw by GrDrawContext.

Modifies GrResourceCache::notifyFlushOccurred() to take a param indicating whether it triggered the
flush that just occurred.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2307053002

Committed: https://skia.googlesource.com/skia/+/1dbb207babecdae8f1f74ed9d9900c73064df744
Review-Url: https://codereview.chromium.org/2307053002