The FileSelectHelper class is using RenderViewHost notifications to
ensure it cleans up itself. However, as I moved it to use
RenderFrameHost for file selection (but not directory enumeration yet),
it opened it up to use-after-free conditions as it was not monitoring
for those objects going away.

This CL fixes that and adds a test to ensure it gets properly cleaned up
when an iframe that requested the file chooser is removed.

BUG=621076

Review-Url: https://codereview.chromium.org/2102883002
Cr-Commit-Position: refs/heads/master@{#402683}