Remove feature flag for disabling Win10 custom titlebar

This flag was just for debugging so it is more appropriate as a
command line switch. Also, this function is called in hot code, and
feature flag queries are slow. A command line switch is safer to
cache; tests are allowed to enable or disable feature flags on the
fly, hence caching a feature flag lookup breaks this contract.

This allows for caching the result without breaking expectations.

Bug: 960913
Change-Id: Ibb9f9b7b7db7b9329984d71318c7488534e04f9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1600494Reviewed-by: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659125}

chrome/browser/win/titlebar_config.cc

@@ -9,17 +9,17 @@
 #include "chrome/common/chrome_switches.h"
 #include "ui/native_theme/native_theme_win.h"
 
-// Enables custom-drawing the titlebar and tabstrip background so that we can
-// paint it various gray colors instead of the default #FFFFFF.
-const base::Feature kWindows10CustomTitlebar{"Windows10CustomTitlebar",
-                                             base::FEATURE_ENABLED_BY_DEFAULT};
-
 bool ShouldCustomDrawSystemTitlebar() {
+  // Cache flag lookup.
+  static const bool custom_titlebar_disabled =
+      base::CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kDisableWindows10CustomTitlebar);
+
   // TODO(bsep): We would like the custom-draw the titlebar in high-contrast
   // mode so that we can correctly draw the caption buttons on the left in RTL
   // mode. But they require a different style and color selection that isn't
   // currently implemented.
-  return !ui::NativeTheme::GetInstanceForNativeUi()->UsesHighContrastColors() &&
-         base::FeatureList::IsEnabled(kWindows10CustomTitlebar) &&
+  return !custom_titlebar_disabled &&
+         !ui::NativeTheme::GetInstanceForNativeUi()->UsesHighContrastColors() &&
          base::win::GetVersion() >= base::win::Version::WIN10;
 }

chrome/common/chrome_switches.cc

@@ -797,6 +797,10 @@ const char kMakeChromeDefault[] = "make-chrome-default";
 #endif  // defined(OS_MACOSX)
 
 #if defined(OS_WIN)
+// Disables custom-drawing the window titlebar on Windows 10.
+const char kDisableWindows10CustomTitlebar[] =
+    "disable-windows10-custom-titlebar";
+
 // Fallback to XPS. By default connector uses CDD.
 const char kEnableCloudPrintXps[]           = "enable-cloud-print-xps";
 

chrome/common/chrome_switches.h

@@ -242,6 +242,7 @@ extern const char kMakeChromeDefault[];
 #endif  // defined(OS_MACOSX)
 
 #if defined(OS_WIN)
+extern const char kDisableWindows10CustomTitlebar[];
 extern const char kEnableCloudPrintXps[];
 extern const char kEnableProfileShortcutManager[];
 extern const char kHideIcons[];