Fixed memory leaks in EditContext tests

When |ContextDestroyed| was called in the InputMethodController,
the active EditContext stored in the InputMethodController wasn't
getting destroyed so explicitly setting it to null in |ContextDestroyed|.

Bug: 1024161
Change-Id: I49489f90b13aa105c2ae6dffdaae19546e31f290
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925891Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Anupam Snigdha <snianu@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#717230}

third_party/blink/renderer/core/editing/ime/input_method_controller.cc

@@ -425,6 +425,7 @@ void InputMethodController::Clear() {
 void InputMethodController::ContextDestroyed(Document*) {
   Clear();
   composition_range_ = nullptr;
+  active_edit_context_ = nullptr;
 }
 
 void InputMethodController::DidAttachDocument(Document* document) {

third_party/blink/web_tests/LeakExpectations

@@ -84,8 +84,6 @@ crbug.com/934144 [ Linux ] http/tests/devtools/tracing/timeline-misc/timeline-fl
 # Sheriff 2019-04-10
 crbug.com/951453 [ Linux ] external/wpt/html/user-activation/activation-transfer-cross-origin-with-click.sub.tentative.html [ Pass Failure ]
 
-crbug.com/1024161 [ Linux ] editing/input/edit-context.html [ Pass Failure ]
-
 # Sheriff 2019-08-21
 crbug.com/996235 [ Linux ] media/controls/doubletap-to-jump-backwards-at-start.html [ Pass Failure ]
 crbug.com/996235 [ Linux ] virtual/audio-service/media/controls/doubletap-to-jump-backwards-at-start.html [ Pass Failure ]