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The previous implementation of ToolbarView had a constraint that
attempted to layout the view's height by constraining its bottom anchor
to kToolbarHeight away from the top layout guide provided by the BVC.
This is incorrect because when the page is scrolled, fullscreen will
translate the toolbar up so that its bottom edge is less than its
height away from the bottom of the top safe area inset.

This CL updates ToolbarView to use |intrinsicContentSize| to specify
kToolbarHeight as its minimum size (enforced via implicit compression
resistance constraints created by returning a custom value for
|intrinsicContentSize|).  The remainder of the toolbar's layout is
managed by constraints created by BVC:
1. |primaryToolbarHeightConstraint|, which accounts for the top layout
   guide's height.
2. |primaryToolbarOffsetConstraint|, which manages the vertical
   positioning of the toolbar.  When a fullscreen progress value is
   observed, BVC updates this value to move the toolbar up and down.
   Note that the height never changes.
3. Leading and trailing constraints that match that of BVC's view so
   the toolbar's width spans the entire screen.

Bug: 799907, 799117, 798496
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I6cf2c7e09d3a8324be4008f7be7c313231fabd43
Reviewed-on: https://chromium-review.googlesource.com/882135
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532175}