While screencasting a tab, do not disable rendering updates while hidden.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 5c5289b355df8115e71cd8525d1d033727f47589..933004a224d587ce1880beb34af675a7192d9dc1 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -309,7 +309,7 @@ WebContentsImpl::WebContentsImpl(
       upload_size_(0),
       upload_position_(0),
       displayed_insecure_content_(false),
-      capturing_contents_(false),
+      capturer_count_(0),
       is_being_destroyed_(false),
       notify_disconnection_(false),
       dialog_manager_(NULL),
@@ -1008,8 +1008,17 @@ bool WebContentsImpl::DisplayedInsecureContent() const {
   return displayed_insecure_content_;
 }
 
-void WebContentsImpl::SetCapturingContents(bool cap) {
-  capturing_contents_ = cap;
+void WebContentsImpl::IncrementCapturerCount() {
+  ++capturer_count_;
+  DVLOG(1) << "There are now " << capturer_count_
+           << " capturing(s) of WebContentsImpl@" << this;
+}
+
+void WebContentsImpl::DecrementCapturerCount() {
+  --capturer_count_;
+  DVLOG(1) << "There are now " << capturer_count_
+           << " capturing(s) of WebContentsImpl@" << this;
+  DCHECK_LE(0, capturer_count_);
 }
 
 bool WebContentsImpl::IsCrashed() const {
@@ -1076,9 +1085,11 @@ void WebContentsImpl::WasShown() {
 }
 
 void WebContentsImpl::WasHidden() {
-  if (!capturing_contents_) {
+  // If there are entities capturing screenshots or video (e.g., mirroring),
+  // don't activate the "disable rendering" optimization.
+  if (capturer_count_ == 0) {
     // |GetRenderViewHost()| can be NULL if the user middle clicks a link to
-    // open a tab in then background, then closes the tab before selecting it.
+    // open a tab in the background, then closes the tab before selecting it.
     // This is because closing the tab calls WebContentsImpl::Destroy(), which
     // removes the |GetRenderViewHost()|; then when we actually destroy the
     // window, OnWindowPosChanged() notices and calls WasHidden() (which