Made page zoom handling work with smooth scroll devices

This CL is a smaller set of the CL https://codereview.chromium.org/688253002/
It fixes some broken functionality exposed by that CL.

R=avi
BUG=384970

Review URL: https://codereview.chromium.org/1554253004

Cr-Commit-Position: refs/heads/master@{#367850}

AUTHORS

@@ -616,6 +616,7 @@ Wesley Lancel <wesleylancel@gmail.com>
 Wesley Wigham <t-weswig@microsoft.com>
 Wesley Wigham <wwigham@gmail.com>
 Will Hirsch <chromium@willhirsch.co.uk>
+Will Shackleton <w.shackleton@gmail.com>
 William Xie <william.xie@intel.com>
 Xiang Long <xiang.long@intel.com>
 Xiaolei Yu <dreifachstein@gmail.com>

content/browser/web_contents/web_contents_impl.cc

@@ -6,6 +6,7 @@
 
 #include <stddef.h>
 
+#include <cmath>
 #include <utility>
 
 #include "base/command_line.h"
@@ -396,6 +397,7 @@ WebContentsImpl::WebContentsImpl(BrowserContext* browser_context)
       closed_by_user_gesture_(false),
       minimum_zoom_percent_(static_cast<int>(kMinimumZoomFactor * 100)),
       maximum_zoom_percent_(static_cast<int>(kMaximumZoomFactor * 100)),
+      zoom_scroll_remainder_(0),
       render_view_message_source_(NULL),
       render_frame_message_source_(NULL),
       fullscreen_widget_routing_id_(MSG_ROUTING_NONE),
@@ -1587,7 +1589,14 @@ bool WebContentsImpl::HandleWheelEvent(
   if (delegate_ && event.wheelTicksY &&
       (event.modifiers & blink::WebInputEvent::ControlKey) &&
       !event.canScroll) {
-    delegate_->ContentsZoomChange(event.wheelTicksY > 0);
+    // Count only integer cumulative scrolls as zoom events; this handles
+    // smooth scroll and regular scroll device behavior.
+    zoom_scroll_remainder_ += event.wheelTicksY;
+    int whole_zoom_scroll_remainder_ = std::lround(zoom_scroll_remainder_);
+    zoom_scroll_remainder_ -= whole_zoom_scroll_remainder_;
+    if (whole_zoom_scroll_remainder_ != 0) {
+      delegate_->ContentsZoomChange(whole_zoom_scroll_remainder_ > 0);
+    }
     return true;
   }
 #endif

content/browser/web_contents/web_contents_impl.h

@@ -1180,6 +1180,9 @@ class CONTENT_EXPORT WebContentsImpl
   int minimum_zoom_percent_;
   int maximum_zoom_percent_;
 
+  // Used to correctly handle integer zooming through a smooth scroll device.
+  float zoom_scroll_remainder_;
+
   // The intrinsic size of the page.
   gfx::Size preferred_size_;