Don't set shownRatio in main thread top controls on animated state change.

third_party/WebKit/Source/core/frame/TopControls.cpp

Index: third_party/WebKit/Source/core/frame/TopControls.cpp
diff --git a/third_party/WebKit/Source/core/frame/TopControls.cpp b/third_party/WebKit/Source/core/frame/TopControls.cpp
index d7d3ad90dfcf318d91f96c0cc1e2f5b159f83821..c44217dde01f4ec2f0d0a9f846d2ffb958934a07 100644
--- a/third_party/WebKit/Source/core/frame/TopControls.cpp
+++ b/third_party/WebKit/Source/core/frame/TopControls.cpp
@@ -95,14 +95,29 @@ void TopControls::setShownRatio(float shownRatio)
     m_frameHost->chromeClient().didUpdateTopControls();
 }
 
-void TopControls::updateConstraints(WebTopControlsState constraints)
+void TopControls::updateConstraintsAndState(
+    WebTopControlsState constraints,
+    WebTopControlsState current,
+    bool animate)
 {
     m_permittedState = constraints;
 
-    if (m_permittedState == WebTopControlsShown)
-        setShownRatio(1.f);
-    else if (m_permittedState == WebTopControlsHidden)
+    DCHECK(!(constraints == WebTopControlsShown && current == WebTopControlsHidden));
+    DCHECK(!(constraints == WebTopControlsHidden && current == WebTopControlsShown));
+
+    // If the change should be animated, let the impl thread drive the change.
+    // Otherwise, immediately set the shown ratio so we don't have to wait for
+    // a commit from the impl thread.
+    if (animate)
+        return;
+
+    if (constraints == WebTopControlsBoth && current == WebTopControlsBoth)
+        return;
+
+    if (constraints == WebTopControlsHidden || current == WebTopControlsHidden)
         setShownRatio(0.f);
+    else
+        setShownRatio(1.f);
 }
 
 void TopControls::setHeight(float height, bool shrinkViewport)