top-chrome slide: Remove unneeded DCHECK

The removed DCHECK didn't make sense. User could
hit Ctrl+Tab to switch tabs while scrolling is in
progress.

TBR=emaxx@chromium.org
BUG=929784

Change-Id: Icd9c26a3650cec610de8c61499e32595347e2e4d
Reviewed-on: https://chromium-review.googlesource.com/c/1459517Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630081}

chrome/browser/ui/views/frame/top_controls_slide_controller_chromeos.cc

@@ -472,7 +472,6 @@ void TopControlsSlideControllerChromeOS::OnTabStripModelChanged(
 
   content::WebContents* new_active_contents = selection.new_contents;
   DCHECK(observed_tabs_.count(new_active_contents));
-  DCHECK(!is_gesture_scrolling_in_progress_);
 
   // Restore the newly-activated tab's shown ratio. If this is a newly inserted
   // tab, its |shown_ratio_| is 1.0f.