omnibox: tweak high contrast scheme for URL readability

The URL's contrast on the selected grey background is too low. This change
increases the brightness of the URL and darkens the background, pushing the
contrast ratio over threshold.

Bug: 870290
Change-Id: I30f40bcf85b283ee1ca6ddf82dd20e3dbeb9d91f
Reviewed-on: https://chromium-review.googlesource.com/1163731Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581203}

chrome/browser/ui/omnibox/omnibox_theme.cc

@@ -249,8 +249,12 @@ SkColor GetOmniboxColor(OmniboxPart part,
       // The spec calls for transparent black (or white) overlays for hover (8%)
       // and select (6%), which can overlap (for 14%). Pre-blend these with the
       // background for the best text AA result.
+      // High contrast mode needs a darker base - Grey 800 with 14% white
+      // overlaid on it (see below) is hard to produce good contrast ratios
+      // against with colors other than white.
       return color_utils::BlendTowardOppositeLuma(
-          dark ? gfx::kGoogleGrey800 : SK_ColorWHITE,
+          dark ? (high_contrast ? gfx::kGoogleGrey900 : gfx::kGoogleGrey800)
+               : SK_ColorWHITE,
           gfx::ToRoundedInt(GetOmniboxStateAlpha(state) * 0xff));
     case OmniboxPart::LOCATION_BAR_TEXT_DEFAULT:
     case OmniboxPart::RESULTS_TEXT_DEFAULT:
@@ -270,6 +274,8 @@ SkColor GetOmniboxColor(OmniboxPart part,
     case OmniboxPart::RESULTS_TEXT_POSITIVE:
       return dark ? gfx::kGoogleGreenDark600 : gfx::kGoogleGreen600;
     case OmniboxPart::RESULTS_TEXT_URL:
+      if (high_contrast)
+        return dark ? gfx::kGoogleBlue300 : gfx::kGoogleBlue700;
       return dark ? gfx::kGoogleBlueDark600 : gfx::kGoogleBlue600;
 
     case OmniboxPart::LOCATION_BAR_BUBBLE_OUTLINE: