Per-host V8 histograms.

content/renderer/render_view_impl.cc

Index: content/renderer/render_view_impl.cc
diff --git a/content/renderer/render_view_impl.cc b/content/renderer/render_view_impl.cc
index 3c1526682498c7356cdf5601f31cba94d9534be8..2a61a12671cabda22cd7ad0fa7290b766253c43e 100644
--- a/content/renderer/render_view_impl.cc
+++ b/content/renderer/render_view_impl.cc
@@ -3171,6 +3171,21 @@ void RenderViewImpl::didCommitProvisionalLoad(WebFrame* frame,
 
   // Check whether we have new encoding name.
   UpdateEncoding(frame, frame->view()->pageEncoding().utf8());
+
+  if (!frame->parent()) {  // Only for top frames.
+    // Check if all RenderViewImpls are displaying a page from the same host,
+    // and update RenderThreadImpl's view on that. If this is the only
+    // RenderViewImpl, the common host is this view's host. If there are many,
+    // check if this one shares the common host of the other
+    // RenderViewImpls. It's ok to not detect some cases where the
+    // RenderViewImpls share a common host.
+    std::string host = GURL(frame->document().url()).host();
+    RenderThreadImpl* render_thread_impl = RenderThreadImpl::current();
+    if (g_view_map.Get().size() == 1)
+      render_thread_impl->SetCommonHost(host);
+    else if (host != render_thread_impl->CommonHost())
+      render_thread_impl->SetCommonHost(std::string());
+  }
 }
 
 void RenderViewImpl::didClearWindowObject(WebFrame* frame) {