cros: Fix black shadow issue for login shelf buttons

Verified it works on a Paine. This is a regression caused by
https://chromium-review.googlesource.com/c/chromium/src/+/1138861

Bug: 873609
Change-Id: I929fb3be9f0f811d901811aebf34c8c52ea42bf2
Reviewed-on: https://chromium-review.googlesource.com/1175192Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583072}

ash/shelf/login_shelf_view.cc

@@ -112,6 +112,8 @@ class LoginShelfButton : public views::LabelButton {
     SetPaintToLayer();
     layer()->SetFillsBoundsOpaquely(false);
 
+    SetTextSubpixelRenderingEnabled(false);
+
     SetImageLabelSpacing(kImageLabelSpacingDp);
     SetTextColor(views::Button::STATE_NORMAL,
                  login_constants::kButtonEnabledColor);