[TTS] Fix reparenting and add Test case.

Updates ContextualSearchTabHelper to handle reparenting correctly,
and improve usage of the mTab member to make that usage more explicit.
    
Adds a new TabObserver method onActivityAttachmentChanged to notify Tabs
when they've been attached or detached from an Activity.

Also refactor mulitwindow test utility methods into a separate utility
class: MultiWindowTestUtils.java, based mostly on static functions
from MultiWindowUtilsTest.java.

BUG=775813, 778540

Change-Id: I460bc3a91494f52aceb77c4880105caadb5c354d
Reviewed-on: https://chromium-review.googlesource.com/738923
Commit-Queue: Donn Denman <donnd@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514328}

• chrome/android/java/src/org/chromium/chrome/browser/contextualsearch/ContextualSearchTabHelper.java[diff]
• chrome/android/java/src/org/chromium/chrome/browser/contextualsearch/SelectionClientManager.java[diff]
• chrome/android/java/src/org/chromium/chrome/browser/tab/EmptyTabObserver.java[diff]
• chrome/android/java/src/org/chromium/chrome/browser/tab/Tab.java[diff]
• chrome/android/java/src/org/chromium/chrome/browser/tab/TabObserver.java[diff]
• chrome/android/java_sources.gni[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/browseractions/BrowserActionActivityTest.java[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/contextualsearch/ContextualSearchManagerTest.java[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/multiwindow/MultiWindowIntegrationTest.java[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/multiwindow/MultiWindowTestHelper.java[Added - diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/multiwindow/MultiWindowUtilsTest.java[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/TabModelMergingTest.java[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/tabmodel/UndoTabModelTest.java[diff]
• content/public/android/java/src/org/chromium/content_public/browser/SelectionClient.java[diff]