Fix swiping down from toolbar to show current tab in new Android tab switcher

We previously had a behavior in the new Android horizontal tab switcher where
opening the tab switcher would show the previous tab instead of the current one.
I previously landed a CL to remove this behavior, but swiping down from the
toolbar is still showing the previous tab. It turns out there's a totally
separate piece of pre-existing logic that, together with the scroll snapping,
is actually causing swiping down from the toolbar to still go to the previous
tab. We of course did not notice this prior to removing the "show previous tab"
behavior.

This CL fixes this pre-existing logic to not show the previous tab.

Bug: 849480,831359
Change-Id: I0b89f87e536c5d4938638fbd6ac7fabde568e772
Reviewed-on: https://chromium-review.googlesource.com/1091230Reviewed-by: Matthew Jones <mdjones@chromium.org>
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565372}

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

@@ -2082,7 +2082,8 @@ public abstract class Stack {
         // Update the scroll offset to put the focused tab at the top.
         final int index = mTabList.index();
 
-        if (mCurrentMode == Orientation.PORTRAIT) {
+        if (mCurrentMode == Orientation.PORTRAIT
+                || ChromeFeatureList.isEnabled(ChromeFeatureList.HORIZONTAL_TAB_SWITCHER_ANDROID)) {
             mScrollOffset = -index * mSpacing;
         } else {
             mScrollOffset = -index * mSpacing + x - LANDSCAPE_SWIPE_DRAG_TAB_OFFSET_DP;