Minor popup blocker cleanups

This CL:
1. Stops null-checking the WebContents when considering to block popups.
   All callers pass in a valid contents already.

2. Tests the case when a caller calls ShowBlockedPopup with an invalid
   id. This seemed like a more reasonable API to be defensive with, but
   it wasn't tested.

Bug: None
Change-Id: I3913ed540b06f57e7fd31cae1f7da7de4da37dab
Reviewed-on: https://chromium-review.googlesource.com/1073847Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562022}

chrome/browser/ui/blocked_content/popup_blocker_browsertest.cc

@@ -332,6 +332,11 @@ IN_PROC_BROWSER_TEST_F(PopupBlockerBrowserTest, PopupPositionMetrics) {
                            static_cast<int>(ListItemPosition::kOnlyItem), 1);
 
   tester.ExpectTotalCount(kClickThroughPosition, 4);
+
+  // Requests to show popups not on the list should do nothing.
+  EXPECT_FALSE(base::ContainsValue(ids, 5));
+  popup_blocker->ShowBlockedPopup(5, disposition);
+  tester.ExpectTotalCount(kClickThroughPosition, 4);
 }
 
 IN_PROC_BROWSER_TEST_F(PopupBlockerBrowserTest, PopupMetrics) {

chrome/browser/ui/blocked_content/popup_blocker_tab_helper.cc

@@ -112,14 +112,13 @@ bool PopupBlockerTabHelper::MaybeBlockPopup(
     NavigateParams* params,
     const content::OpenURLParams* open_url_params,
     const blink::mojom::WindowFeatures& window_features) {
+  DCHECK(web_contents);
   DCHECK(!open_url_params ||
          open_url_params->user_gesture == params->user_gesture);
 
   LogAction(Action::kInitiated);
 
   const bool user_gesture = params->user_gesture;
-  if (!web_contents)
-    return false;
 
   if (base::CommandLine::ForCurrentProcess()->HasSwitch(
           switches::kDisablePopupBlocking)) {