[Toast] Show a toast to press the close tab button again for pinned tabs

Add a toast that asks the user to press the keyboard shortcut to close a
pinned tab again.

Ctrl / Cmd + W closes the tab quickly for power users but it also means
it's easy to accidentally close a pinned tab.

This change will experiment with the permanance of pinned tabs and
whether or not we should add a speed bump when closing pinned tabs.

Functionality can be enabled via:
chrome://flags#pinned-tab-toast-on-close

The toast will only be displayed if the active tab is a pinned tab, the
user activates the keyboard shortcut to close a tab, and we do not have
multiple other tabs selected.

Originally the plan was to allow users to hold the close tab shortcut
but that ran into many technical problems. For now we will keep the
double tap interaction as the intended behavior.

Change-Id: I8d150795cb29df276e3bf39bbd5c9ba7ef5c5dd1
Bug: 409822606, 416316768
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6494675
Reviewed-by: Alison Gale <agale@chromium.org>
Commit-Queue: Darryl James <dljames@chromium.org>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1457322}