Ask client for navigation policy for empty url

This is the only case when we don't even ask the client to decide the
navigation policy, and it breaks a very nice assumption that
DecidePolicyForNavigation is always called for renderer-initiated
navigations.

For reference: this was introduced [1] in 2002 (sic!) and being
copied around since then.

[1] https://chromium.googlesource.com/chromium/src/+/80834d145816195289a30e4168a0659380a00d75%5E%21/

Bug: 789577
Change-Id: Ic4a4671bcc34db46f70d53d4281d2ce68017ec1a
Reviewed-on: https://chromium-review.googlesource.com/1114199Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570856}

third_party/blink/renderer/core/loader/frame_loader.cc

@@ -1336,8 +1336,8 @@ NavigationPolicy FrameLoader::ShouldContinueForNavigationPolicy(
     HTMLFormElement* form,
     mojom::blink::BlobURLTokenPtr blob_url_token,
     bool check_with_client) {
-  // Don't ask if we are loading an empty URL.
-  if (request.Url().IsEmpty() || substitute_data.IsValid())
+  // Don't ask if we already have the data.
+  if (substitute_data.IsValid())
     return kNavigationPolicyCurrentTab;
 
   // Check for non-escaped new lines in the url.