[iOS] Fix InfobarModalOverlayCoordinator's |-modalHeightForWidth:|

If the modal view is a scroll view containing content views whose
|-sizeThatFits:| is not implemented, calling |-sizeThatFits:| on the
scroll view will not calculate the same size that it ultimately used
when the contents are laid out.  This resulted in an incorrect size
used for UITableViews with custom cells whose sizing selectors were not
implemented.  Manually triggering a layout pass enusures that all
view resizing resulting from constraint resolution occurs and the
content size is calculated correctly.  While it is not desirable to
manually trigger layout passes, |-modalHeightForWidth:| is only called
once when the InfobarModalOverlayCoordinator is started, so it should
not have any significant performance implications.

Bug: none
Change-Id: I57dfe692a0f439896a08ef6ddecef048a388d003
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159769
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Chris Lu <thegreenfrog@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761151}