Avoid starting a distillation process if the URL is not distillable.

The distillation does not need to start in this cases, the data
source is going to fallback to the error webpage.

Review URL: https://codereview.chromium.org/1003153003

Cr-Commit-Position: refs/heads/master@{#321328}

chrome/browser/dom_distiller/tab_utils.cc

@@ -110,7 +110,13 @@ void StartNavigationToDistillerViewer(content::WebContents* web_contents,
   web_contents->GetController().LoadURLWithParams(params);
 }
 
-void StartDistillation(content::WebContents* web_contents) {
+void MaybeStartDistillation(content::WebContents* web_contents) {
+  const GURL& last_committed_url = web_contents->GetLastCommittedURL();
+  if (!dom_distiller::url_utils::IsUrlDistillable(last_committed_url)) {
+    delete web_contents;
+    return;
+  }
+
   // Start distillation using |web_contents|, and ensure ViewerHandle stays
   // around until the viewer requests distillation.
   SelfDeletingRequestDelegate* view_request_delegate =
@@ -125,7 +131,6 @@ void StartDistillation(content::WebContents* web_contents) {
       dom_distiller_service->CreateDefaultDistillerPageWithHandle(
                                  source_page_handle.Pass()).Pass();
 
-  const GURL& last_committed_url = web_contents->GetLastCommittedURL();
   scoped_ptr<ViewerHandle> viewer_handle = dom_distiller_service->ViewUrl(
       view_request_delegate, distiller_page.Pass(), last_committed_url);
   view_request_delegate->TakeViewerHandle(viewer_handle.Pass());
@@ -156,5 +161,5 @@ void DistillCurrentPageAndView(content::WebContents* old_web_contents) {
   CoreTabHelper::FromWebContents(old_web_contents)->delegate()->SwapTabContents(
       old_web_contents, new_web_contents, false, false);
 
-  StartDistillation(old_web_contents);
+  MaybeStartDistillation(old_web_contents);
 }