Fix the size of the Autofill popups

The width of the DropdownPopupWindow is updated to match its
AnchoredView. Moreover, the DropdownPopupWindow will be aligned to
the center of the AnchoredView.

BUG=820886

Change-Id: Ia82bd5a78c1e2ab4f1e68aecfa38fa219751de36
Reviewed-on: https://chromium-review.googlesource.com/960568Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Amirhossein Simjour <asimjour@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543614}

ui/android/java/src/org/chromium/ui/DropdownPopupWindow.java

@@ -34,6 +34,7 @@ public class DropdownPopupWindow implements AnchoredPopupWindow.LayoutObserver {
     ListAdapter mAdapter;
     private ListView mListView;
     private Drawable mBackground;
+    private int mHorizontalPadding;
 
     /**
      * Creates an DropdownPopupWindow with specified parameters.
@@ -75,6 +76,12 @@ public class DropdownPopupWindow implements AnchoredPopupWindow.LayoutObserver {
                 new AnchoredPopupWindow(context, mAnchorView, mBackground, mListView, rectProvider);
         mAnchoredPopupWindow.addOnDismissListener(onDismissLitener);
         mAnchoredPopupWindow.setLayoutObserver(this);
+        Rect paddingRect = new Rect();
+        mBackground.getPadding(paddingRect);
+        rectProvider.setInsetPx(0, /* top= */ paddingRect.bottom, 0, /* bottom= */ paddingRect.top);
+        mHorizontalPadding = paddingRect.right + paddingRect.left;
+        mAnchoredPopupWindow.setPreferredHorizontalOrientation(
+                AnchoredPopupWindow.HORIZONTAL_ORIENTATION_CENTER);
     }
 
     /**
@@ -112,9 +119,12 @@ public class DropdownPopupWindow implements AnchoredPopupWindow.LayoutObserver {
         boolean wasShowing = mAnchoredPopupWindow.isShowing();
         mAnchoredPopupWindow.setVerticalOverlapAnchor(false);
         mAnchoredPopupWindow.setHorizontalOverlapAnchor(true);
-        mAnchoredPopupWindow.setMaxWidth(measureContentWidth()
-                + mContext.getResources().getDimensionPixelSize(
-                          R.dimen.dropdown_item_label_margin));
+        int contentWidth = measureContentWidth();
+        if (mAnchorView.getWidth() < contentWidth) {
+            mAnchoredPopupWindow.setMaxWidth(contentWidth + mHorizontalPadding);
+        } else {
+            mAnchoredPopupWindow.setMaxWidth(mAnchorView.getWidth() + mHorizontalPadding);
+        }
         mAnchoredPopupWindow.show();
         mListView.setDividerHeight(0);
         ApiCompatibilityUtils.setLayoutDirection(