Create the base Custom Context Menu Dialog.

chrome/android/java/src/org/chromium/chrome/browser/contextmenu/ContextMenuHelper.java

Index: chrome/android/java/src/org/chromium/chrome/browser/contextmenu/ContextMenuHelper.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/contextmenu/ContextMenuHelper.java b/chrome/android/java/src/org/chromium/chrome/browser/contextmenu/ContextMenuHelper.java
index 615b7c8fb67885ad79fd377688ec0a8c45c7933d..ecc93695a2bdd7b964c54e3888d11eed6b372413 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/contextmenu/ContextMenuHelper.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/contextmenu/ContextMenuHelper.java
@@ -15,6 +15,7 @@ import org.chromium.base.Callback;
 import org.chromium.base.VisibleForTesting;
 import org.chromium.base.annotations.CalledByNative;
 import org.chromium.base.metrics.RecordHistogram;
+import org.chromium.chrome.browser.ChromeFeatureList;
 import org.chromium.chrome.browser.share.ShareHelper;
 import org.chromium.content.browser.ContentViewCore;
 import org.chromium.content_public.browser.WebContents;
@@ -133,17 +134,20 @@ public class ContextMenuHelper implements OnCreateContextMenuListener {
     @Override
     public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {

[Ted C, 2017/03/21 16:29:39]

So this path still relies on View.showContextMenu, which we probably shouldn't
call anymore for the new UI.  I think we should be splitting in showContextMenu
above.

[JJ, 2017/03/22 23:35:32]

Alright, refactored. I did try this before in a previous life where we got rid
of the ContextMenu parameter but it turns out that is actually heavily used and
several tests fail if you don't include that! In order to not duplicate too much
code I've added an extra global for callback. Tell me if this is along your line
of thinking.

In order to get rid of that pesky "menu" we'll have to rewrite dozens of tests.
I know this needs to be done but in 500+ line cl it's something I'd rather have
done in a seperate cl.

         assert mPopulator != null;
-
         List<Pair<Integer, List<ContextMenuItem>>> items =
                 mPopulator.buildContextMenu(menu, v.getContext(), mCurrentContextMenuParams);
-        ContextMenuUi menuUi = new PlatformContextMenuUi(menu);
-        menuUi.displayMenu(mActivity, mCurrentContextMenuParams, items, new Callback<Integer>() {
+        Callback<Integer> callback = new Callback<Integer>() {
             @Override
             public void onResult(Integer result) {
                 mPopulator.onItemSelected(
                         ContextMenuHelper.this, mCurrentContextMenuParams, result);
             }
-        });
+        };
+
+        ContextMenuUi menuUi = (ChromeFeatureList.isEnabled(ChromeFeatureList.CUSTOM_CONTEXT_MENU))
+                ? new CustomContextMenuDialog()
+                : new PlatformContextMenuUi(menu);
+        menuUi.displayMenu(mActivity, mCurrentContextMenuParams, items, callback);
     }
 
     /**