Abandon prerenders upon render thread shutdown.
Have PrerenderDispatcher abandon its current prerenders when
the renderer is about to shut down -- doing so later isn't
supported as Blink is about to disappear.
Along with this, on the Blink side, weaken the reference that
a Prerender object keep to its PrerenderClient. The latter
should keep the former alive, but not vice versa -- if no one
is referring to the client within Blink, it can be GCed.
This avoids prolonged retention of prerendering Blink objects.
R=
BUG=604325
Review URL: https://codereview.chromium.org/1900583002
Cr-Commit-Position: refs/heads/master@{#387912}
3 files changed
