color_utils cleanup:

* Attempt to define and standardize usage of "luma" vs. "(relative) luminance" vs. "lightness", at least as far as color_utils functions, comments, and direct callers are concerned.  Especially the first two terms are often both called "luminance" (universally, not just in Chrome code), so the distinction is confusing.
* Misc. cleanups to make caller code shorter or clearer.
* Naming consistency: remove "Get" from function names where we're not doing some simple "getter" operation, but rather computing a value.
* Modify some caller behaviors to be hopefully-more-correct
* Note that GetReadableColor() can accept values with alpha, it will just ignore the alpha.
* Add a PickContrastingColor() function, which is basically a subset of GetReadableColor().  This will be useful for an upcoming change where we want to choose between two fixed colors instead of luma-inverting a single input.
* Use more precise multipliers in Luma().

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/1761183002

Cr-Commit-Position: refs/heads/master@{#379643}

1 files changed, 2 insertions, 5 deletions

diff --git a/ash/frame/default_header_painter.cc b/ash/frame/default_header_painter.cc
index 432764b..18086ac 100644
--- a/ash/frame/default_header_painter.cc
+++ b/ash/frame/default_header_painter.cc
@@ -42,8 +42,6 @@ const SkColor kHeaderContentSeparatorInactiveColor =
 const SkColor kDefaultFrameColor = SkColorSetRGB(242, 242, 242);
 // Duration of crossfade animation for activating and deactivating frame.
 const int kActivationCrossfadeDurationMs = 200;
-// Luminance below which to use white caption buttons.
-const int kMaxLuminanceForLightButtons = 125;
 
 // Tiles an image into an area, rounding the top corners.
 void TileRoundRect(gfx::Canvas* canvas,
@@ -279,9 +277,8 @@ void DefaultHeaderPainter::PaintHeaderContentSeparator(gfx::Canvas* canvas) {
 }
 
 bool DefaultHeaderPainter::ShouldUseLightImages() {
-  int luminance = color_utils::GetLuminanceForColor(
-      mode_ == MODE_INACTIVE ? inactive_frame_color_ : active_frame_color_);
-  return luminance < kMaxLuminanceForLightButtons;
+  return color_utils::IsDark(mode_ == MODE_INACTIVE ? inactive_frame_color_
+                                                    : active_frame_color_);
 }
 
 void DefaultHeaderPainter::UpdateAllButtonImages() {