[ios] Prevent more taps in the NTP if the underlying NTP hasn't committed.

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.  Additionally block any most visited
action, the fake omnibox and voice search 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: 931306

Change-Id: Iff2c11eadaeed60407879d51b5b2577e606c8a18
Reviewed-on: https://chromium-review.googlesource.com/c/1464174
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631376}
3 files changed