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When Activity is restarted (either after configuration change, unless
handled manually, or after being killed due to memory pressure), the
system FragmentManager restores all the Fragments during Activity's
onCreate event. The current APIs make it impossible to properly
create a BrowserFragment without an instance of WebLayer, and that
prohibits asynchronous WebLayer initialization.

There are two ways to deal with it:
- Disable it in a hacky way: the embedder runs
savedInstanceState.remove("android:support:fragments");
in their activity prior to super.onCreate(). The
embedder should then manually save and restore the parameters of
each fragment they have.
- Handle the restoration the "correct" way.

More detailed discussion:

This CL enables the second path. This means putting BrowserFragment
on the top of dependency tree, which is opposite to what we have
now (getWebLayer().createProfile().createBrowserFragment()). In
this CL, BrowserFragment is created independently, and later
attached to WebLayer instance.

I suggest making BrowserFragment the only client-side object that
can temporarily exist without the weblayer counterpart.
All other objects should have the same lifecycle on both sides.

This is achieved by making all browser APIs accessible only through
BrowserFragmentController, which in turn can be accessed only after
the embedder attaches BrowserFragment to WebLayer.

More detailed discussion:
https://docs.google.com/document/d/15xCcdO8V1tBYH-UJStmi0aEzLXcUWDkc-KWI_Wb7rdQ

This CL doesn't enable async init of WebLayer after process restart: trying to
retrieve BrowserFragmentController or attach BrowserFragment blocks the thread
if WebLayer hasn't been already initialized, and in case of process restart it
can't have been initialized.

Change-Id: Ibe6fd369e8d60af5cedc256d3ebf1db210b579bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829082Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Pavel Shmakov <pshmakov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704695}