Fix regression: empty tooltip appears in Linux and Chrome OS

ui/views/corewm/tooltip_controller.cc

Index: ui/views/corewm/tooltip_controller.cc
diff --git a/ui/views/corewm/tooltip_controller.cc b/ui/views/corewm/tooltip_controller.cc
index a52c806a9928e56dd9a4f212c9535e1e456da06d..96d44204d9732e0d7a4d7fa1fc61a43c9e34bde8 100644
--- a/ui/views/corewm/tooltip_controller.cc
+++ b/ui/views/corewm/tooltip_controller.cc
@@ -327,7 +327,7 @@ void TooltipController::UpdateIfRequired() {
 }
 
 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();