Implement "Look Up In Dictionary" context menu item asynchronously.

content/browser/renderer_host/render_widget_host_view_mac.mm

Index: content/browser/renderer_host/render_widget_host_view_mac.mm
diff --git a/content/browser/renderer_host/render_widget_host_view_mac.mm b/content/browser/renderer_host/render_widget_host_view_mac.mm
index 9500038092b8c8630da89909a8f4d2ae95452169..f4925cef60bd1043fc954e3e87314bfbd9f1e678 100644
--- a/content/browser/renderer_host/render_widget_host_view_mac.mm
+++ b/content/browser/renderer_host/render_widget_host_view_mac.mm
@@ -2355,10 +2355,9 @@ void RenderWidgetHostViewMac::OnDisplayMetricsChanged(
   }
 }
 
-// This is invoked only on 10.8 or newer when the user taps a word using
-// three fingers.
-- (void)quickLookWithEvent:(NSEvent*)event {
-  NSPoint point = [self convertPoint:[event locationInWindow] fromView:nil];
+- (void)showLookUpDictionaryOverlayAtPoint:(NSPoint)point {
+  // The PDF plugin does not support getting the attributed string at point.
+  // TODO: support GetStringAtPoint() for PDF. See crbug.com/152438.
   TextInputClientMac::GetInstance()->GetStringAtPoint(
       renderWidgetHostView_->render_widget_host_,
       gfx::Point(point.x, NSHeight([self frame]) - point.y),
@@ -2373,6 +2372,13 @@ void RenderWidgetHostViewMac::OnDisplayMetricsChanged(
   );
 }
 
+// This is invoked only on 10.8 or newer when the user taps a word using
+// three fingers.
+- (void)quickLookWithEvent:(NSEvent*)event {
+  NSPoint point = [self convertPoint:[event locationInWindow] fromView:nil];
+  [self showLookUpDictionaryOverlayAtPoint:point];
+}
+
 // This method handles 2 different types of hardware events.
 // (Apple does not distinguish between them).
 //  a. Scrolling the middle wheel of a mouse.