assistant: update icon colors

Increase the contrast for a11y.

Bug: 910560
Test: locally build and run
Change-Id: I1b8bb4d000902cd2db7349a4ca6cc63fd02f6853
Reviewed-on: https://chromium-review.googlesource.com/c/1366324Reviewed-by: Tao Wu <wutao@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614751}

ash/assistant/util/views_util.h

@@ -32,7 +32,7 @@ views::ImageButton* CreateImageButton(views::ButtonListener* listener,
                                       int icon_size_in_dip,
                                       int accessible_name_id,
                                       AssistantButtonId button_id,
-                                      SkColor icon_color = gfx::kGoogleGrey600);
+                                      SkColor icon_color = gfx::kGoogleGrey700);
 
 }  // namespace util
 }  // namespace assistant