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This change adds an experimental entry-point for the tab search
feature in the tab strip.

Currently this tab search button instance has been implemented as
another instance of the NewTabButton class, positioned directly next
to the existing new tab button.

The addition of this feature is gated by both the ENABLE_TAB_SEARCH
build flag and the kTabSearch UI feature flag.

Outside of changes gated by the build and feature flags, this CL
wraps the existing new tab button in a |new_tab_button_wrapper_|.
This wrapper allows the addition of the tab search button without
introducing significant changes to existing logic that deals with the
new tab button (ie animation, hit testing, padding etc). This wrapper
is not expected to change the current behavior of the new tab button.

There is follow up work to fix issues such as visual style and
creating a base class inherited by both the actual new tab button
and tab search button.

Bug: 1099917
Change-Id: I63cd0804603a8d25efd4416e4f3ea4307ff6535c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2333341
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795969}