Remove definition of the DEFINE_WEB_CONTENTS_USER_DATA_KEY macro.

The macro is a no-op since
https://chromium-review.googlesource.com/c/chromium/src/+/1093015.

Bug: 589840
Change-Id: I4fe4479d5dbfe9116032933bcde8c8889be0ebd2
Reviewed-on: https://chromium-review.googlesource.com/1165644Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581355}

content/public/browser/web_contents_user_data.h

@@ -25,9 +25,6 @@ namespace content {
 //   friend class content::WebContentsUserData<FooTabHelper>;
 //   // ... more private stuff here ...
 // }
-// --- in foo_tab_helper.cc ---
-// DEFINE_WEB_CONTENTS_USER_DATA_KEY(FooTabHelper);
-//
 template <typename T>
 class WebContentsUserData : public base::SupportsUserData::Data {
  public:
@@ -58,10 +55,6 @@ class WebContentsUserData : public base::SupportsUserData::Data {
   }
 };
 
-// Macro previously used to define the UserDataKey().
-// TODO(fdoray): Remove this. https://crbug.com/589840
-#define DEFINE_WEB_CONTENTS_USER_DATA_KEY(TYPE)
-
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_BROWSER_WEB_CONTENTS_USER_DATA_H_

docs/tab_helpers.md

@@ -63,8 +63,6 @@ class TitleLoggerTabHelper
 
   DISALLOW_COPY_AND_ASSIGN(TitleLoggerTabHelper);
 };
-
-DEFINE_WEB_CONTENTS_USER_DATA_KEY(TitleLoggerTabHelper);
 ```
 
 We want each tab to have this `WebContentsObserver` attached to it, so that it