Increase Disabled Text Contrast of Menus, Buttons, and Textfields

From the available palette, this is the closest we can get to 4 without
making the text actually look enabled.

The contrast ratio of menus and textfields go from 2.61 to 3.68 and
the contrast ratio of buttons go from 2.54 to 3.54.

BUG=872524
TBR=pkasting@chromium.org
Color change with accessibility approval.

Change-Id: I3d5c917d5b4be193a78e950d021d97dceb4ab15d
Reviewed-on: https://chromium-review.googlesource.com/c/1353947Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612527}

ui/native_theme/common_theme.cc

@@ -85,7 +85,7 @@ SkColor GetAuraColor(NativeTheme::ColorId color_id,
   }
 
   // Shared constant for disabled text.
-  constexpr SkColor kDisabledTextColor = SkColorSetRGB(0xA1, 0xA1, 0x92);
+  constexpr SkColor kDisabledTextColor = gfx::kGoogleGrey600;
 
   // Buttons:
   constexpr SkColor kButtonEnabledColor = gfx::kChromeIconGrey;