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I have made a truly wonderful mess of WebViewPlugin, via:
https://chromium.googlesource.com/chromium/src/+/8489f1bf78c2c6f296e0df6c529751d29e8a0188
https://chromium.googlesource.com/chromium/src/+/8ed4bf69eca5e2696aeab731a8e9a15e8f379f2b

My fundamental misunderstanding (I think!) was regarding what
WebViewPlugin is listening to via the various interfaces it subclasses.
It uses WebFrameClient and WebViewClient to observe its internal
WebView, while it uses WebPlugin and RenderViewObserver to interact
with the real external page. In the patches above, I mixed and matched
messages from those 2 sources because I thought they were coming from
the same context.

This patch reverts the damage done in those patches, and restores
ReplayReceivedData, more-or-less the way it was before I
started breaking stuff. It then continues the work I started in
https://chromium.googlesource.com/chromium/src/+/8e1badd5fbc054cf4ffc9303e8cb0aa22ce55ca6
and moves the subclassing of WebViewClient to a private helper class.
This helper, renamed to WebViewHelper, now handles all callbacks coming
from the internal WebView (i.e., the "WebView" in "WebViewPlugin"), and
WebViewPlugin continues subclassing WebPlugin and RenderViewObserver
to observe the outside world. This will hopefully make it harder to
screw things up in the same way in the future.

BUG=675585

Review-Url: https://codereview.chromium.org/2600253003
Cr-Commit-Position: refs/heads/master@{#441401}