Remove unused WebContentsImpl::is_notifying_observers_ variable

CL https://crrev.com/c/2343269 excludes its usage.
Looks it's left accidently.

Change-Id: I6ce13710f55ea74267456fd2c5edb3d803ca948d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2512893Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Nasko Oskov <nasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823768}

content/browser/web_contents/web_contents_impl.h

@@ -1822,10 +1822,6 @@ class CONTENT_EXPORT WebContentsImpl : public WebContents,
   // See getter above.
   bool is_being_destroyed_;
 
-  // Keep track of whether this WebContents is currently iterating over its list
-  // of observers, during which time it should not be deleted.
-  bool is_notifying_observers_;
-
   // Indicates whether we should notify about disconnection of this
   // WebContentsImpl. This is used to ensure disconnection notifications only
   // happen if a connection notification has happened and that they happen only