Fix regression: empty tooltip appears in Linux and Chrome OS

ui/views/corewm/tooltip_controller.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/views/corewm/tooltip_controller.h"
 
 #include <stddef.h>
 
 #include <utility>
 #include <vector>
@@ skipping 309 matching lines @@
                                                   kDelayForTooltipUpdateInMs),
                                    this, &TooltipController::ShowTooltip);
       }
     } else {
       ShowTooltip();  // Allow tooltip to show up without delay for unit tests.
     }
   }
 }
 
 void TooltipController::ShowTooltip() {
-  if (!tooltip_window_)
+  if (!tooltip_window_ || tooltip_text_whitespace_trimmed_.empty())

[sky, 2017/01/25 23:43:21]

Also, how do we end up here with an empty tooltip?

[chengx, 2017/01/26 01:16:24]

Because this function is called from UpdateIfRequired() above but with a delay
of 500 ms. In UpdateIfRequired(), tooltip_text_whitespace_trimmed_ is checked
first. If the string is empty, then ShowTooltip() won't be called. However,
since the ShowTooltip() function call is delayed with 500 ms,
tooltip_text_whitespace_trimmed_ can be set to empty by other event handlers
during that period. So we need to double check if string is empty here. 

     return;
   gfx::Point widget_loc =
       curr_mouse_loc_ + tooltip_window_->GetBoundsInScreen().OffsetFromOrigin();
   tooltip_->SetText(tooltip_window_, tooltip_text_whitespace_trimmed_,
                     widget_loc);
   tooltip_->Show();
   int timeout = GetTooltipShownTimeout();
   if (timeout > 0) {
     tooltip_shown_timer_.Start(FROM_HERE,
                                base::TimeDelta::FromMilliseconds(timeout), this,
@@ skipping 38 matching lines @@
     return;
   if (tooltip_window_)
     tooltip_window_->RemoveObserver(this);
   tooltip_window_ = target;
   if (tooltip_window_)
     tooltip_window_->AddObserver(this);
 }
 
 }  // namespace corewm
 }  // namespace views