Use CONTEXT_BUTTON_MD for Dice promo buttons

One might ask: why do even have this lever? I agree, so I filed
https://crbug.com/918911

Bug: 918194
Change-Id: I0ea816a50d840e71bc6f8c5684877c8353cb0c11
Reviewed-on: https://chromium-review.googlesource.com/c/1394734Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619748}

chrome/browser/ui/views/sync/dice_signin_button_view.cc

@@ -36,8 +36,7 @@ DiceSigninButtonView::DiceSigninButtonView(
   // Regular MD text button when there is no account.
   views::MdTextButton* button = views::MdTextButton::Create(
       button_listener,
-      l10n_util::GetStringUTF16(IDS_PROFILES_DICE_SIGNIN_BUTTON),
-      views::style::CONTEXT_BUTTON);
+      l10n_util::GetStringUTF16(IDS_PROFILES_DICE_SIGNIN_BUTTON));
   button->SetProminent(prominent);
   AddChildView(button);
   signin_button_ = button;
@@ -101,8 +100,7 @@ DiceSigninButtonView::DiceSigninButtonView(
                      views::GridLayout::USE_PREF, 0, 0);
   views::MdTextButton* button = views::MdTextButton::Create(
       button_listener,
-      l10n_util::GetStringUTF16(IDS_PROFILES_DICE_SIGNIN_BUTTON),
-      views::style::CONTEXT_BUTTON);
+      l10n_util::GetStringUTF16(IDS_PROFILES_DICE_SIGNIN_BUTTON));
   button->SetProminent(true);
   grid_layout->AddView(button);
   signin_button_ = button;