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This stuff was a LOT of plumbing, all to eventually determine "is the
current tab an NTP?". That can be had MUCH simpler :)

Before:
SearchTabHelper (per-tab) has a SearchModel and updates that with the
"isNTP" state. The state for the active tab gets propagated via
SearchDelegate to BrowserInstantController's SearchModel instance.
InstantController listens for changes to that, and on non-NTP->NTP
transitions sends the "Instant info" (i.e. MV tiles and theme) to the
Instant process.

After:
InstantController watches for relevant navigations itself and just
checks the "isNTP" state directly. Several kilometers of plumbing are
removed, and there is much rejoicing.

Manually tested the following cases:
- Navigating regular page <-> remote NTP
- Navigating regular page <-> local NTP
- Navigating local NTP <-> remote NTP
All these for regular and forward/back navigations, and for Google and
third-party DSE.

Bug: 627747
Change-Id: Id55b1f28550b4aeee1e5472ac0e8eebd0ee0d65c
Reviewed-on: https://chromium-review.googlesource.com/678719Reviewed-by: Chris Pickel <sfiera@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504342}