Remove overlay interstitial methods from webcontents

Removes ShowingInterstitialPage from WebContents, and
DidAttachInterstitialPage and DidDetachInterstitialPage from
WebContentsObserver. Cleans up call sites.

Bug: 1077074

[email protected]

Change-Id: Ia139d07d4ba7fd8ef8a2afff8981b049d61a5045
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2147921
Commit-Queue: Carlos IL <[email protected]>
Reviewed-by: Nasko Oskov <[email protected]>
Reviewed-by: Sadrul Chowdhury <[email protected]>
Reviewed-by: Sergey Ulanov <[email protected]>
Reviewed-by: Clemens Arbesser <[email protected]>
Reviewed-by: Nate Fischer <[email protected]>
Reviewed-by: Andrew Grieve <[email protected]>
Reviewed-by: Mike Wittman <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Auto-Submit: Carlos IL <[email protected]>
Cr-Commit-Position: refs/heads/master@{#769475}
diff --git a/content/public/browser/web_contents.h b/content/public/browser/web_contents.h
index 039f001..0244a69 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -291,10 +291,7 @@
 
   // Gets the virtual URL currently being displayed in the URL bar, if there is
   // one. This URL might be a pending navigation that hasn't committed yet, so
-  // it is not guaranteed to match the current page in this WebContents. A
-  // typical example of this is interstitials, which show the URL of the
-  // new/loading page (active) but the security context is of the old page (last
-  // committed).
+  // it is not guaranteed to match the current page in this WebContents.
   virtual const GURL& GetVisibleURL() = 0;
 
   // Gets the virtual URL of the last committed page in this WebContents.
@@ -769,11 +766,6 @@
   // is true when using Shift-Tab).
   virtual void FocusThroughTabTraversal(bool reverse) = 0;
 
-  // Interstitials -------------------------------------------------------------
-
-  // Various other systems need to know about our interstitials.
-  virtual bool ShowingInterstitialPage() = 0;
-
   // Misc state & callbacks ----------------------------------------------------
 
   // Check whether we can do the saving page operation this page given its MIME