Switch tabs on mouse down instead of mouse up.

This is consistent with other Mac apps and with Views, and makes the tab
strip feel more responsive.

Bug: 799230
Change-Id: I45a375f5892c10e5886e18f27ba049eb6215a17f
Reviewed-on: https://chromium-review.googlesource.com/895925Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533373}

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

@@ -63,6 +63,9 @@ static BOOL PointIsInsideView(NSPoint screenPoint, NSView* view) {
 - (void)maybeStartDrag:(NSEvent*)theEvent forTab:(TabController*)tab {
   [self resetDragControllers];
 
+  // Activate the tab.
+  [tab selectTab:self];
+
   // Resolve overlay back to original window.
   sourceWindow_ = [[tab view] window];
   if ([sourceWindow_ isKindOfClass:[NSPanel class]]) {
@@ -136,7 +139,6 @@ static BOOL PointIsInsideView(NSPoint screenPoint, NSView* view) {
     } else if (type == NSLeftMouseDragged) {
       [self continueDrag:theEvent];
     } else if (type == NSLeftMouseUp) {
-      [tab selectTab:self];
       [self endDrag:theEvent];
       break;
     } else {