Add about:flags support for doc.write script blocking.

third_party/WebKit/Source/core/loader/FrameFetchContext.cpp

Index: third_party/WebKit/Source/core/loader/FrameFetchContext.cpp
diff --git a/third_party/WebKit/Source/core/loader/FrameFetchContext.cpp b/third_party/WebKit/Source/core/loader/FrameFetchContext.cpp
index 5ad8728cef79d720c136425206b56954b23c7b6e..4d9aa8d0da1474b6e3d64ccafe1ae229880cdb4a 100644
--- a/third_party/WebKit/Source/core/loader/FrameFetchContext.cpp
+++ b/third_party/WebKit/Source/core/loader/FrameFetchContext.cpp
@@ -75,6 +75,43 @@
 
 namespace blink {
 
+namespace {

[Nate Chapin, 2016/04/12 16:53:15]

Why this namespace?

[Bryan McQuade, 2016/04/12 17:28:38]

Oh, I'm accustomed to using an anonymous namespace for functions in the C++
file, per the C++ style guide
https://google.github.io/styleguide/cppguide.html#Namespaces

I do see some code in blink that does this as well.

I can remove the anonymous namespace if you prefer - let me know.

+
+bool ShouldDisallowFetchForMainFrameScript(const ResourceRequest& request, FetchRequest::DeferOption defer, const Document& document)

[Nate Chapin, 2016/04/12 16:53:15]

Nit: shouldDisallowFetchForMainFrameScript

[Bryan McQuade, 2016/04/12 17:28:38]

Done

+{
+    // Only scripts inserted via document.write are candidates for having their
+    // fetch disallowed.
+    if (!document.isInDocumentWrite()) {

[Nate Chapin, 2016/04/12 16:53:15]

Here and below, no {} for ifs with single-line bodies.

[Bryan McQuade, 2016/04/12 17:28:38]

Done

+        return false;
+    }
+
+    if (!document.settings()) {
+        return false;
+    }
+
+    const bool isSlowConnection = networkStateNotifier().connectionType() == WebConnectionTypeCellular2G;
+    const bool disallowFetch = document.settings()->disallowFetchForDocWrittenScriptsInMainFrame() || (document.settings()->disallowFetchForDocWrittenScriptsInMainFrameOnSlowConnections() && isSlowConnection);
+    if (!disallowFetch) {
+        return false;
+    }
+
+    // Only block synchronously loaded (parser blocking) scripts.
+    if (defer != FetchRequest::NoDefer) {
+        return false;
+    }
+
+    // Avoid blocking same origin scripts, as they may be used to render main
+    // page content, whereas cross-origin scripts inserted via document.write
+    // are likely to be third party content.
+    if (request.url().host() == document.getSecurityOrigin()->domain()) {
+        return false;
+    }
+
+    return true;
+}
+
+} // namespace
+
 FrameFetchContext::FrameFetchContext(DocumentLoader* loader, Document* document)
     : m_document(document)
     , m_documentLoader(loader)
@@ -205,27 +242,8 @@ WebCachePolicy FrameFetchContext::resourceRequestCachePolicy(const ResourceReque
     // For users on slow connections, we want to avoid blocking the parser in
     // the main frame on script loads inserted via document.write, since it can
     // add significant delays before page content is displayed on the screen.
-    if (type == Resource::Script && isMainFrame()) {
-        const bool isInDocumentWrite = m_document && m_document->isInDocumentWrite();
-        const bool disallowFetchForDocWriteScripts = frame()->settings() && frame()->settings()->disallowFetchForDocWrittenScriptsInMainFrame();
-
-        if (isInDocumentWrite && disallowFetchForDocWriteScripts) {
-            // only synchronously loaded scripts should be blocked
-            const bool isSync = defer == FetchRequest::NoDefer;
-
-            // Not blocking same origin scripts as they may be used to render main page content
-            // whereas cross-origin scripts inserted via document.write are likely
-            // for third party content.
-            const bool isThirdParty = request.url().host() != m_document->getSecurityOrigin()->domain();
-
-            // Only blocking in slow connections where the performance penalty is worst case.
-            // For now we restrict slow connections to 2G, in future this might be expanded using the
-            // network quality estimator.
-            const bool isSlowConnection = networkStateNotifier().connectionType() == WebConnectionTypeCellular2G;
-
-            if (isSync && isThirdParty && isSlowConnection)
-                return WebCachePolicy::ReturnCacheDataDontLoad;
-        }
+    if (type == Resource::Script && isMainFrame() && m_document && ShouldDisallowFetchForMainFrameScript(request, defer, *m_document)) {
+        return WebCachePolicy::ReturnCacheDataDontLoad;
     }
 
     if (request.isConditional())