Remove most use of TabModelSelector and ChromeActivity in ReaderModeManager

This patch replaces most usages of the TabModelSelector with Tab to
facilitate the transition of the ReaderModeManager to be a per-tab
object. The patch also includes a few other mechanical changes around
accessing the web contents and removes some now unnecessary null
checks.

Bug: 1069815, 1051184
Change-Id: Ica37a9781fd299494403174f46aa5b3a9c27939e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2147675
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759812}

chrome/android/java/src/org/chromium/chrome/browser/ChromeActivity.java

@@ -746,7 +746,7 @@ public abstract class ChromeActivity<C extends ChromeActivityComponent>
         }
 
         if (ReaderModeManager.isEnabled(this)) {
-            mReaderModeManager = new ReaderModeManager(getTabModelSelector(), this);
+            mReaderModeManager = new ReaderModeManager(getTabModelSelector());
         }
 
         TraceEvent.end("ChromeActivity:CompositorInitialization");

chrome/android/java/src/org/chromium/chrome/browser/infobar/ReaderModeInfoBar.java

@@ -89,7 +89,7 @@ public class ReaderModeInfoBar extends InfoBar {
     @Override
     public void onCloseButtonClicked() {
         if (getReaderModeManager() != null) {
-            getReaderModeManager().onClosed(StateChangeReason.CLOSE_BUTTON);
+            getReaderModeManager().onClosed(getTab(), StateChangeReason.CLOSE_BUTTON);
         }
         super.onCloseButtonClicked();
     }