Fix Performance::now() for browser navigations.

Source/core/loader/DocumentLoadTiming.cpp

Index: Source/core/loader/DocumentLoadTiming.cpp
diff --git a/Source/core/loader/DocumentLoadTiming.cpp b/Source/core/loader/DocumentLoadTiming.cpp
index 6204ca613575cfa5214b40a644c225e5ac9384de..3d7edd2a1d51f8d0da6532b220d9df9f677c5750 100644
--- a/Source/core/loader/DocumentLoadTiming.cpp
+++ b/Source/core/loader/DocumentLoadTiming.cpp
@@ -74,7 +74,18 @@ void DocumentLoadTiming::markNavigationStart()
 void DocumentLoadTiming::setNavigationStart(double navigationStart)
 {
     ASSERT(m_referenceMonotonicTime && m_referenceWallTime);
+
+    // This should be used only to indicate that the navigation started in the
+    // embedder before the renderer was created.
+    ASSERT(navigationStart <= m_navigationStart);
     m_navigationStart = navigationStart;
+
+    // |m_referenceMonotonicTime| and |m_referenceWallTime| represent
+    // navigationStart. When the embedder sets navigationStart (because the
+    // navigation started earlied on the browser side), we need to adjust these
+    // as well.
+    m_referenceWallTime = monotonicTimeToPseudoWallTime(navigationStart);
+    m_referenceMonotonicTime = navigationStart;
 }
 
 void DocumentLoadTiming::addRedirect(const KURL& redirectingUrl, const KURL& redirectedUrl)