Remove DEFINE_WEB_CONTENTS_USER_DATA_KEY in //headless.

It is no longer to invoke the DEFINE_WEB_CONTENTS_USER_DATA_KEY macro
to define a global variable from which the key of a
WebContentsUserData is derived. The key is now derived from a static
local variable.

This CL was uploaded by git cl split.

R=irisu@chromium.org

Bug: 589840
Change-Id: I5bcbc1394aef4188b7b703685bcc0a1ffa5c3428
Reviewed-on: https://chromium-review.googlesource.com/1150285Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581228}

headless/lib/browser/headless_print_manager.cc

@@ -18,8 +18,6 @@
 #include "printing/print_job_constants.h"
 #include "printing/units.h"
 
-DEFINE_WEB_CONTENTS_USER_DATA_KEY(headless::HeadlessPrintManager);
-
 namespace headless {
 
 struct HeadlessPrintManager::FrameDispatchHelper {