Since TabManager is aware of TabModels (for tracking which tabs are actually visible, recently selected, etc) we need a TabModelObserver on all platforms. One exists on Android, but there's no native equivalent.

TabManager uses the TabStripModelObserver to track changes to audibility state. This is very round-about and it should be directly trackable via WebContentsObserver.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/55393bbffd92a6de2333bd6c14af12948eddbb33

commit 55393bbffd92a6de2333bd6c14af12948eddbb33
Author: Chris Hamilton <chrisha@chromium.org>
Date: Thu May 10 01:14:28 2018

Create native TabModelObserver.

This adds a native TabModelObserver to
chrome/browser/ui/android/tab_model, along with the necessary plumbing
and JNI bridge.

BUG=836409

Change-Id: I0b9bff5146d163705756ff5cbe4c6ed1f211cc35
Reviewed-on: https://chromium-review.googlesource.com/1038611
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557405}
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModel.java
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelImpl.java
[add] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelObserverJniBridge.java
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/android/java_sources.gni
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/BUILD.gn
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/BUILD.gn
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model.h
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model_jni_bridge.cc
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model_jni_bridge.h
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model_list_unittest.cc
[add] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model_observer.cc
[add] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model_observer.h
[add] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model_observer_jni_bridge.cc
[add] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model_observer_jni_bridge.h
[modify] https://crrev.com/55393bbffd92a6de2333bd6c14af12948eddbb33/chrome/browser/ui/android/tab_model/tab_model_unittest.cc

Note that a bunch of code paths that are currently desktop only (TabStripModelObserver) should be made cross-platform by having a platform-agnostic equivalent of TabStripModel. This allows knowing which WebContents are directly related to tabs in the UI, for example.