Eliminate USE_ASH macro and replace with OS_CHROMEOS

We have not shipped Windows Ash for years. There is a 1:1 mapping
between USE_ASH and OS_CHROMEOS, so consolidate them.

The BUILD.gn files will be cleaned up separately.

Bug: 673826
Test: compiles, gn check on Chrome OS, Linux and Linux Ozone
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I5ebf64a133d1032f0d7864ed1a180d0f0b170da1
Reviewed-on: https://chromium-review.googlesource.com/692699
Commit-Queue: James Cook <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Cr-Commit-Position: refs/heads/master@{#506410}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 5d26322..c24fc964 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1685,7 +1685,7 @@
 
 #endif  // BUILDFLAG(ENABLE_WEBRTC)
 
-#if defined(USE_ASH)
+#if defined(OS_CHROMEOS)
 
 extern const char kAshDisableSmoothScreenRotationName[];
 extern const char kAshDisableSmoothScreenRotationDescription[];
@@ -1723,7 +1723,7 @@
 extern const char kUiSlowAnimationsName[];
 extern const char kUiSlowAnimationsDescription[];
 
-#endif  // defined(USE_ASH)
+#endif  // defined(OS_CHROMEOS)
 
 #if defined(TOOLKIT_VIEWS) || defined(OS_ANDROID)