[Bedrock] Move SigninViewController to BrowserWindowFeatures

This CL moves SigninViewController off Browser and into
BrowserWindowFeatures.

Stored Profile*, TabStripModel* and BrowserWindowInterface* as member
variables in SigninViewController instead of browser.

BrowserWindowInterface was necessary as member variable because:
1. SigninViewController code calls Navigate which needs Browser.
2. SigninViewController code also calls various other classes which
   need Browser too.

Moving 1 off Browser is being tracked separately.
Moving 2 off Browser will be also be tracked as a future follow up.

TearDownPreBrowserViewDestruction function was added in
SigninViewController to handle some browser test failures. And
CloseModalSignin call was moved into that.

Previously the browser tests depended on receiving OnBrowserRemoved
message from Browser destruction before SigninViewController was
destroyed to ensure "browser-is-valid". With this refactoring
SigninViewController is destroyed before that message is sent. So,
an explicit TearDownPreBrowserViewDestruction call is necessary.

TearDownPreBrowserViewDestruction is now called deterministically from
BrowserWindowFeatures::TearDownPreBrowserViewDestruction(). It is also
called from BrowserWithTestWindowTest::TearDown to handle issues with
some unit tests.

A nogncheck was added to resolve a circular dependency problem and a
tracking bug was created to fix it later.

Also opportunitistically made some variables const.

Bug: 418802319

Change-Id: Id0db9f9446bfeef69fb4cd73a1b70d978a529687
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6604286
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Lukaszewicz <tluk@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1472804}