DecidePolicyForNavigation should only be called for content-initiated

Adding CHECKs to verify that we did not miss any cases when it's called
for browser-initiated navigation. Will cleanup after a couple of Canaries.

Bug: 789577

Change-Id: I36890e1cb653a7f1795a187be0d80f2a99e10aaf
Reviewed-on: https://chromium-review.googlesource.com/1135934
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575371}

content/renderer/render_frame_impl.cc

@@ -5937,6 +5937,13 @@ WebNavigationPolicy RenderFrameImpl::DecidePolicyForNavigation(
                             ->navigation_state()
                             ->IsContentInitiated()
                       : !IsBrowserInitiated(pending_navigation_params_.get());
+  // TODO(dgozman): clean this up after some Canary coverage.
+  CHECK(!pending_navigation_params_);
+  if (info.extra_data) {
+    CHECK(static_cast<DocumentState*>(info.extra_data)
+              ->navigation_state()
+              ->IsContentInitiated());
+  }
 
   // Webkit is asking whether to navigate to a new URL.
   // This is fine normally, except if we're showing UI from one security