Small cleanup for double-tap to zoom on desktop

We update/remove comments that claim that double-tap to zoom is specific
to Android.

We also enable double-tap to zoom on Mac via WebPreferences rather than
hardcoding it as always enabled.

Bug: 802369
Change-Id: Ib78a0a85f295166bff3363ed65dceca9e5e3468e
Reviewed-on: https://chromium-review.googlesource.com/1235129Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Rick Byers <rbyers@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593557}

content/public/common/web_preferences.cc

@@ -187,12 +187,15 @@ WebPreferences::WebPreferences()
       user_gesture_required_for_presentation(true),
       text_track_margin_percentage(0.0f),
       immersive_mode_enabled(false),
+#if defined(OS_ANDROID) || defined(OS_MACOSX)
+      double_tap_to_zoom_enabled(true),
+#else
+      double_tap_to_zoom_enabled(false),
+#endif
 #if !defined(OS_ANDROID)
       text_autosizing_enabled(false),
-      double_tap_to_zoom_enabled(false),
 #else
       text_autosizing_enabled(true),
-      double_tap_to_zoom_enabled(true),
       font_scale_factor(1.0f),
       device_scale_adjustment(1.0f),
       force_enable_zoom(false),

content/public/common/web_preferences.h

@@ -229,10 +229,10 @@ struct CONTENT_EXPORT WebPreferences {
 
   bool immersive_mode_enabled;
 
-  bool text_autosizing_enabled;
-
   bool double_tap_to_zoom_enabled;
 
+  bool text_autosizing_enabled;
+
 #if defined(OS_ANDROID)
   float font_scale_factor;
   float device_scale_adjustment;

content/renderer/render_view_impl.cc

@@ -1009,7 +1009,6 @@ void RenderView::ApplyWebPreferences(const WebPreferences& prefs,
   }
 
 #if defined(OS_MACOSX)
-  settings->SetDoubleTapToZoomEnabled(true);
   web_view->SetMaximumLegibleScale(prefs.default_maximum_page_scale_factor);
 #endif
 

third_party/blink/public/platform/web_input_event.h

@@ -162,12 +162,14 @@ class WebInputEvent {
     // because it may still turn into a GestureDoubleTap.
     kGestureTapUnconfirmed,
 
-    // Double-tap is two single-taps spread apart in time, like a double-click.
-    // This event is only sent on desktop pages viewed on an Android phone, and
-    // is always preceded by GestureTapUnconfirmed.  It's an instruction to
-    // Blink to perform a PageScaleAnimation zoom onto the double-tapped
-    // content.  (It's treated differently from GestureTap with tapCount=2,
-    // which can also happen.)
+    // On Android, double-tap is two single-taps spread apart in time, like a
+    // double-click. This event is only sent on desktop pages, and is always
+    // preceded by GestureTapUnconfirmed. It's an instruction to Blink to
+    // perform a PageScaleAnimation zoom onto the double-tapped content. (It's
+    // treated differently from GestureTap with tapCount=2, which can also
+    // happen.)
+    // On desktop, this event may be used for a double-tap with two fingers on
+    // a touchpad, as the desired effect is similar to Android's double-tap.
     kGestureDoubleTap,
 
     kGestureTypeLast = kGestureDoubleTap,

third_party/blink/renderer/core/exported/web_view_impl.cc

@@ -543,9 +543,6 @@ WebInputEventResult WebViewImpl::HandleGestureEvent(
         AnimateDoubleTapZoom(
             FlooredIntPoint(scaled_event.PositionInRootFrame()));
       }
-      // GestureDoubleTap is currently only used by Android for zooming. For
-      // WebCore, GestureTap with tap count = 2 is used instead. So we drop
-      // GestureDoubleTap here.
       event_result = WebInputEventResult::kHandledSystem;
       WidgetClient()->DidHandleGestureEvent(event, event_cancelled);
       return event_result;