DevTools: fix rule insertion with file URLs.

This patch allows accessing rules for inspector style sheets.
Our tests didn't catch the regression since we run with
--allow-file-access-from-files flag.

Bug: 814937
Change-Id: I58eabb8e299a81c5a912d5a42175ab78ad904704
Reviewed-on: https://chromium-review.googlesource.com/935749Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539166}

third_party/WebKit/Source/core/css/CSSStyleSheet.cpp

@@ -301,6 +301,8 @@ bool CSSStyleSheet::CanAccessRules() const {
   Document* document = OwnerDocument();
   if (!document)
     return true;
+  if (document->GetStyleEngine().InspectorStyleSheet() == this)
+    return true;
   if (document->GetSecurityOrigin()->CanRequest(base_url))
     return true;
   if (allow_rule_access_from_origin_ &&