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On iPads, when many tabs are open, additional tabs opened in the background
(through the context menu) will collapse together and remain hidden. Users will
have to manually scroll through to see newly opened ones.

This CL fixes the issue by enabling the scenario that every time a new tab is
inserted, we auto scroll the tab strip view to make it immediately visible.

BUG=228419

Review-Url: https://codereview.chromium.org/2611003002
Cr-Commit-Position: refs/heads/master@{#442106}