[Mac] Make new tab button animation RTL-aware

chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm

Index: chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
diff --git a/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm b/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
index d380384d8778ccfec5f89e8513b72c61afc6de5f..97eb5f57a8c69e4dc60dc633054ae5fa214548fe 100644
--- a/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
+++ b/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
@@ -1180,14 +1180,17 @@ CGFloat FlipXInView(NSView* view, CGFloat width, CGFloat x) {
       [self setNewTabButtonHoverState:shouldShowHover];
 
       // Move the new tab button into place. We want to animate the new tab
-      // button if it's moving to the left (closing a tab), but not when it's
-      // moving to the right (inserting a new tab). If moving right, we need
+      // button if it's moving back (closing a tab), but not when it's
+      // moving forward (inserting a new tab). If moving forward, we need
       // to use a very small duration to make sure we cancel any in-flight
       // animation to the left.
       if (visible && animate) {
         ScopedNSAnimationContextGroup localAnimationGroup(true);
-        BOOL movingLeft = NSMinX(newTabNewFrame) < NSMinX(newTabTargetFrame_);
-        if (!movingLeft) {
+        BOOL movingBack = NSMinX(newTabNewFrame) < NSMinX(newTabTargetFrame_);
+        if (cocoa_l10n_util::ShouldDoExperimentalRTLLayout())
+          movingBack = !movingBack;
+
+        if (!movingBack) {
           localAnimationGroup.SetCurrentContextShortestDuration();
         }
         [[newTabButton_ animator] setFrame:newTabNewFrame];