Reland: Fix a leak of a document that is held by AutofillAgent

This CL is just relanding https://chromium-review.googlesource.com/c/chromium/src/+/597482,
after confirming that there are no statistical significant regressions
in the real world by Finch experiments (crbug.com/755489).

Note that this might still cause Telemetry regressions reported at
crbug.com/753071 before, but this is expected.

A document is leaked because AutofillAgent hold a lastly used element
even after navigation happens, and the document of the element remains
until a new (input) element is focused. This CL fixes this issue by
resetting the element in AutofillAgent when navigation happens.

Bug: 734427, 755489
Change-Id: Ie3ee7f202abf56e22102e3d7f68cf2778f85ca87
Reviewed-on: https://chromium-review.googlesource.com/908371
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539105}

components/autofill/content/renderer/autofill_agent.cc

@@ -192,16 +192,7 @@ void AutofillAgent::DidCommitProvisionalLoad(bool is_new_navigation,
     return;
 
   // Navigation to a new page or a page refresh.
-
-  // Do Finch testing to see how much regressions are caused by this leak fix
-  // (crbug/753071).
-  std::string group_name =
-      base::FieldTrialList::FindFullName("FixDocumentLeakInAutofillAgent");
-  if (base::StartsWith(group_name, "enabled",
-                       base::CompareCase::INSENSITIVE_ASCII)) {
-    element_.Reset();
-  }
-
+  element_.Reset();
   form_cache_.Reset();
   ResetLastInteractedElements();
   OnFormNoLongerSubmittable();