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Sometimes the underlying ios/web page used for the NTP (about://newtab)
takes a long time to load.  Loading any page before the newtab is committed
will leave ios/web in a bad state.  Instead, block any most visited and
content suggestion taps until the underlying NTP is committed.

See: crbug.com/925304 for more context.  Remove this when ios/web supports
queueing multiple loads during this state.

Bug: 929148
Change-Id: I78bf8c58127c19f96432e68510c1565ad4b6d973
Reviewed-on: https://chromium-review.googlesource.com/c/1456645Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629808}