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LayoutManagerBase::GetAvailableSize(host, child) will now do the
following:
 - as before, if the cached layout is still valid since the last call to
   Layout(), it will return the available size of |child| in |host|'s
   layout
 - if the layout has been invalidated since the last call to Layout(), a
   new cached layout for |host| is calculated, which will be used in the
   next call to Layout() assuming the layout is not invalidated again

This gives us two guarantees we did not have before:
 1. instead of returning [no bounds], a correct size bound will almost
    always be returned
 2. a safeguard is in place to ensure that we don't get a stale value
    during |host|'s layout calculation - in fact, we no longer allow the
	method to be called during |host|'s layout calculation (it will
	DCHECK())

In general it is legal to call GetAvailableSize() during |child|'s
Layout() - which happens after |host|'s layout is committed - but not
during, for example, a FlexRule computation. It doesn't make sense for a
FlexRule (or similar) to call GetAvailableSize() because it already
receives a size bound from the caller.

This also fixes a potential issue (related to crbug.com/1041614) which
could result in a view calling layout code in response to certain types
of widget setup (among other possible situations) before the layout's
child data is updated, resulting in a DCHECK() or crash in the layout
manager. (For at least the short term an example crash can be found here:
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8858322112097966752/+/steps/unit_tests__with_patch_/0/logs/Deterministic_failure:_PasswordSaveUpdateWithAccountStoreViewTest.HasTitleAndTwoButtons__status_CRASH_/0)

Change-Id: I3e491c3cb7a648867597ecf830fe52d11f483419
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622720
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842653}