Clear FrameProgressTracker before detaching from the embedder.

FrameProgressTracker can trigger a progress complete notification when
detaching frames. If it sends the final progress complete notification
for the page, then it will try to call back into the embedder, so this
must be handled before detaching from the embedder.

BUG=332652

Review URL: https://codereview.chromium.org/130743003

git-svn-id: svn://svn.chromium.org/blink/trunk@164810 bbb929c8-8fbe-4397-9dbb-9b2b20218538