Update trackpad pinch-zoom code for corrected scale semantics

content/browser/web_contents/web_contents_impl.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 "content/browser/web_contents/web_contents_impl.h"
 
 #include <utility>
 
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
@@ skipping 1245 matching lines @@
     // at ..., -3*step, -2*step, -step, step, 2*step, 3*step, ... but not at 0)
     // so that it's easier to get back to your starting point than it is to
     // overshoot.
     float nextStep = (abs(currentPinchZoomStepDelta_) + 1) * kZoomStepValue;
     float backStep = abs(currentPinchZoomStepDelta_) * kZoomStepValue;
     float zoomInThreshold = (currentPinchZoomStepDelta_ >= 0) ? nextStep
         : -backStep;
     float zoomOutThreshold = (currentPinchZoomStepDelta_ <= 0) ? -nextStep
         : backStep;
 
-    totalPinchGestureAmount_ += event.data.pinchUpdate.scale;
+    totalPinchGestureAmount_ += (event.data.pinchUpdate.scale - 1.0);
     if (totalPinchGestureAmount_ > zoomInThreshold) {
       currentPinchZoomStepDelta_++;
       if (delegate_)
         delegate_->ContentsZoomChange(true);
     } else if (totalPinchGestureAmount_ < zoomOutThreshold) {
       currentPinchZoomStepDelta_--;
       if (delegate_)
         delegate_->ContentsZoomChange(false);
     }
     return true;
@@ skipping 2616 matching lines @@
 
 void WebContentsImpl::OnPreferredSizeChanged(const gfx::Size& old_size) {
   if (!delegate_)
     return;
   const gfx::Size new_size = GetPreferredSize();
   if (new_size != old_size)
     delegate_->UpdatePreferredSize(this, new_size);
 }
 
 }  // namespace content