ForEachObserver -> NotifyObservers to save binary size

Using function pointers rather than lambdas to address binary
size regression from the introduction of ForEachObserver.
Recovers 11,468 of the original 15,600 regression.

Bug: 1116531

Change-Id: I56352c559931859610b00a7dfad615052147ffbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2639396
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845967}

content/browser/web_contents/web_contents_impl.h

@@ -1443,14 +1443,15 @@ class CONTENT_EXPORT WebContentsImpl : public WebContents,
     void AddObserver(WebContentsObserver* observer);
     void RemoveObserver(WebContentsObserver* observer);
 
-    template <class ForEachCallable>
-    void ForEachObserver(const ForEachCallable& callable) {
-      TRACE_EVENT0("content", "Iterating over WebContentsObservers");
+    // T1 must be a pointer to a WebContentsObserver method.
+    template <typename T1, typename... P1>
+    void NotifyObservers(T1 func, P1&&... args) {
+      TRACE_EVENT0("content", "WebContentsObserverList::NotifyObservers");
       base::AutoReset<bool> scope(&is_notifying_observers_, true);
       for (WebContentsObserver& observer : observers_) {
         TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("content.verbose"),
                      "Dispatching WebContentsObserver callback");
-        callable(&observer);
+        ((observer).*(func))(std::forward<P1>(args)...);
       }
     }