Add a null check in WebLocalFrameImpl::SetIsolatedWorldSecurityOrigin()

https://chromium-review.googlesource.com/752006 added an IsolatedCopy()
call but the SecurityOrigin can be null when
WebLocalFrameImpl::SetIsolatedWorldSecurityOrigin() is called from
TestRunnerForSpecificView::SetIsolatedWorldSecurityOrigin().

Bug: 793649, 779730
Change-Id: I017eec29cc6c8cc9a753181c9b6f335569cccb9f
Reviewed-on: https://chromium-review.googlesource.com/825653
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524542}

third_party/WebKit/Source/core/frame/WebLocalFrameImpl.cpp

@@ -666,7 +666,8 @@ void WebLocalFrameImpl::SetIsolatedWorldSecurityOrigin(
     const WebSecurityOrigin& security_origin) {
   DCHECK(GetFrame());
   DOMWrapperWorld::SetIsolatedWorldSecurityOrigin(
-      world_id, security_origin.Get()->IsolatedCopy());
+      world_id,
+      security_origin.Get() ? security_origin.Get()->IsolatedCopy() : nullptr);
 }
 
 void WebLocalFrameImpl::SetIsolatedWorldContentSecurityPolicy(