Move MHTML file writing out of the renderer main thread.

third_party/WebKit/public/web/WebFrameSerializer.h

Index: third_party/WebKit/public/web/WebFrameSerializer.h
diff --git a/third_party/WebKit/public/web/WebFrameSerializer.h b/third_party/WebKit/public/web/WebFrameSerializer.h
index ff2ac88c4cc9a6da95e35063f28d329d24f8cfaa..777935916787776e35e88f9497c7eb913e60426f 100644
--- a/third_party/WebKit/public/web/WebFrameSerializer.h
+++ b/third_party/WebKit/public/web/WebFrameSerializer.h
@@ -37,6 +37,7 @@
 #include "../platform/WebURL.h"
 #include "../platform/WebVector.h"
 #include "WebFrameSerializerCacheControlPolicy.h"
+#include <vector>
 
 namespace blink {
 
@@ -76,7 +77,7 @@ public:
     // Same |boundary| needs to used for all generateMHTMLHeader and
     // generateMHTMLParts and generateMHTMLFooter calls that belong to the same
     // MHTML document (see also rfc1341, section 7.2.1, "boundary" description).
-    BLINK_EXPORT static WebData generateMHTMLHeader(
+    BLINK_EXPORT static std::vector<char> generateMHTMLHeader(
         const WebString& boundary, WebLocalFrame*, MHTMLPartsGenerationDelegate*);
 
     // Generates and returns MHTML parts for the given frame and the
@@ -85,7 +86,7 @@ public:
     // Same |boundary| needs to used for all generateMHTMLHeader and
     // generateMHTMLParts and generateMHTMLFooter calls that belong to the same
     // MHTML document (see also rfc1341, section 7.2.1, "boundary" description).
-    BLINK_EXPORT static WebData generateMHTMLParts(
+    BLINK_EXPORT static std::vector<char> generateMHTMLParts(
         const WebString& boundary, WebLocalFrame*, MHTMLPartsGenerationDelegate*);
 
     // Generates and returns an MHTML footer.
@@ -93,7 +94,7 @@ public:
     // Same |boundary| needs to used for all generateMHTMLHeader and
     // generateMHTMLParts and generateMHTMLFooter calls that belong to the same
     // MHTML document (see also rfc1341, section 7.2.1, "boundary" description).
-    BLINK_EXPORT static WebData generateMHTMLFooter(const WebString& boundary);
+    BLINK_EXPORT static std::vector<char> generateMHTMLFooter(const WebString& boundary);
 
     // IMPORTANT:
     // The API below is an older implementation of frame serialization that