Handle swipes with no preceding scroll events

I incorrectly assumed that any ET_GESTURE_SWIPE would be preceded by an
ET_GESTURE_SCROLL_BEGIN. Sometimes this is not the case, and this caused a
DCHECK.

This change handles swipes that weren't preceded by a scroll event.

Bug: None
Change-Id: I535e43a10e6f6162080af4ba4b85a8fefc8e2cd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128091Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754717}

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

@@ -201,7 +201,11 @@ class WebUITabStripContainerView::DragToOpenHandler : public ui::EventHandler {
         // sent after. From logging, it seems like ET_GESTURE_SCROLL_END
         // is sometimes also sent after this. It will be ignored here
         // since |drag_in_progress_| is set to false.
-        DCHECK(drag_in_progress_);
+        if (!drag_in_progress_) {
+          // If a swipe happens quickly enough, scroll events might not
+          // have been sent. Tell the container a drag began.
+          container_->UpdateHeightForDragToOpen(0.0f);
+        }
         container_->EndDragToOpen(event->details().swipe_down());
         event->SetHandled();
         drag_in_progress_ = false;