Allow image documents in MHT if there is only one resource.

third_party/WebKit/Source/platform/mhtml/MHTMLArchive.cpp

Index: third_party/WebKit/Source/platform/mhtml/MHTMLArchive.cpp
diff --git a/third_party/WebKit/Source/platform/mhtml/MHTMLArchive.cpp b/third_party/WebKit/Source/platform/mhtml/MHTMLArchive.cpp
index cca842e5ccd52da9bb7b2e4f0fee3ac363c6f0c4..6a852fe6faf774d5f03b5e928127c9697bdf0309 100644
--- a/third_party/WebKit/Source/platform/mhtml/MHTMLArchive.cpp
+++ b/third_party/WebKit/Source/platform/mhtml/MHTMLArchive.cpp
@@ -78,16 +78,34 @@ MHTMLArchive* MHTMLArchive::create(const KURL& url,
     return nullptr;  // Invalid MHTML file.
 
   MHTMLArchive* archive = new MHTMLArchive;
+
+  size_t resourcesCount = resources.size();
   // The first document suitable resource is the main resource of the top frame.
-  for (size_t i = 0; i < resources.size(); ++i) {
-    const AtomicString& mimeType = resources[i]->mimeType();
-    if (archive->mainResource() ||
-        !MIMETypeRegistry::isSupportedNonImageMIMEType(mimeType) ||
-        MIMETypeRegistry::isSupportedJavaScriptMIMEType(mimeType) ||
-        mimeType == "text/css")
+  for (size_t i = 0; i < resourcesCount; ++i) {
+    if (archive->mainResource()) {
       archive->addSubresource(resources[i].get());
-    else
+      continue;
+    }
+
+    const AtomicString& mimeType = resources[i]->mimeType();
+    bool isMimeTypeSuitableForMainResource =
+        MIMETypeRegistry::isSupportedNonImageMIMEType(mimeType);
+    // Want to allow image-only MHTML archives, but retain behavior for other
+    // documents that have already been created expecting the first HTML page to
+    // be considered the main resource.

[Łukasz Anforowicz, 2016/10/18 00:46:41]

I thought that MHTML spec requires the first part to be the main resource, but
after rereading rfc2557, section 7 I am not so sure.  At any rate, I guess
keeping the old behavior (i.e. treating the first text/html part as the main
resource) makes sense (i.e. I don't see a compelling reason to start questioning
this old behavior).

+    if (resourcesCount == 1 &&
+        MIMETypeRegistry::isSupportedImageResourceMIMEType(mimeType)) {
+      isMimeTypeSuitableForMainResource = true;
+    }
+    // explicitly disallow JS and CSS as the main resource.
+    if (MIMETypeRegistry::isSupportedJavaScriptMIMEType(mimeType) ||
+        MIMETypeRegistry::isSupportedStyleSheetMIMEType(mimeType))
+      isMimeTypeSuitableForMainResource = false;
+
+    if (isMimeTypeSuitableForMainResource)
       archive->setMainResource(resources[i].get());
+    else
+      archive->addSubresource(resources[i].get());
   }
   return archive;
 }