[Extensions] Remove GetAssociatedWebContents() calls from passwordsPrivate

The ExtensionFunction::GetAssociatedWebContentsDeprecated() method is
unpredictable and can return surprising values in certain circumstances.
It is deprecated and should be removed to ensure that callers are using
the WebContents they expect.

Update the passwordsPrivate API to use GetSenderWebContents() instead.

Bug: 461394
Change-Id: I24b75989786d06aa14a64f403d134e293eb9902a
Reviewed-on: https://chromium-review.googlesource.com/1040530Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555708}

chrome/browser/extensions/api/passwords_private/passwords_private_api.cc

@@ -196,7 +196,7 @@ PasswordsPrivateExportPasswordsFunction::Run() {
       base::BindOnce(
           &PasswordsPrivateExportPasswordsFunction::ExportRequestCompleted,
           this),
-      GetAssociatedWebContentsDeprecated());
+      GetSenderWebContents());
   return RespondLater();
 }