Change StackLayout#setActiveStackState() to take an int

StackLayout#setActiveStackState() is used by StackLayout to set the currently
active tab stack. This method currently takes a boolean (true for the incognito
stack, false for the normal stack). This CL changes this method to instead take
an integer representing an index, so we can support more than two tab stacks.

This CL also changes this method to return void (previously it was returning a
boolean: true if the active stack was actually changed, and false if it wasn't),
since the return value is never actually used.

Bug: 648314
Change-Id: Ief119b4a1a92f8345b2b3556f9003cf4e3092f61
Reviewed-on: https://chromium-review.googlesource.com/957274
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542266}

chrome/android/java/src/org/chromium/chrome/browser/compositor/layouts/phone/StackLayout.java

@@ -285,7 +285,7 @@ public class StackLayout extends Layout implements Animatable<StackLayout.Proper
                     nextIndex = currentIndex - 1;
                 }
                 if (mStacks.get(nextIndex).isDisplayable()) {
-                    setActiveStackState(nextIndex == 1);
+                    setActiveStackState(nextIndex);
                 }
             }
 
@@ -1081,7 +1081,7 @@ public class StackLayout extends Layout implements Animatable<StackLayout.Proper
      */
     private void flingStacks(int index) {
         // velocityX is measured in pixel per second.
-        setActiveStackState(index == INCOGNITO_STACK_INDEX);
+        setActiveStackState(index);
         finishScrollStacks();
         requestStackUpdate();
     }
@@ -1226,13 +1226,10 @@ public class StackLayout extends Layout implements Animatable<StackLayout.Proper
     /**
      * Sets the active tab stack.
      *
-     * @param isIncognito True if the model to select is incognito.
-     * @return Whether the tab stack index passed in differed from the currently selected stack.
+     * @param stackIndex Index of the tab stack to be made active.
      */
-    public boolean setActiveStackState(boolean isIncognito) {
-        if (isIncognito == mTabModelSelector.isIncognitoSelected()) return false;
-        mTemporarySelectedStack = isIncognito ? INCOGNITO_STACK_INDEX : NORMAL_STACK_INDEX;
-        return true;
+    public void setActiveStackState(int stackIndex) {
+        mTemporarySelectedStack = stackIndex;
     }
 
     private void resetScrollData() {