--- a/build/build_config.h
+++ b/build/build_config.h
@@ -16,6 +16,40 @@
 #ifndef BUILD_BUILD_CONFIG_H_
 #define BUILD_BUILD_CONFIG_H_
 
+// A brief primer on #defines:
+//
+// - __ANDROID__ is automatically defined by the Android toolchain (see
+//   https://goo.gl/v61lXa). It's not defined when building host code.
+// - __ANDROID_HOST__ is defined via -D by Android.mk when building host code
+//   within an Android checkout.
+// - ANDROID is defined via -D when building code for either Android targets or
+//   hosts. Use __ANDROID__ and __ANDROID_HOST__ instead.
+// - OS_ANDROID is a Chrome-specific define used to build Chrome for Android
+//   within the NDK.
+
+// Android targets and hosts don't use tcmalloc.
+#if defined(__ANDROID__) || defined(__ANDROID_HOST__)
+#define NO_TCMALLOC
+#endif  // defined(__ANDROID__) || defined(__ANDROID_HOST__)
+
+// Use the Chrome OS version of the code for both Android targets and Chrome OS builds.
+#if !defined(__ANDROID_HOST__)
+#define OS_CHROMEOS 1
+#endif  // !defined(__ANDROID_HOST__)
+
+#if defined(__ANDROID__)  // Android targets
+
+#define __linux__ 1
+
+#elif !defined(__ANDROID_HOST__)  // Chrome OS
+
+// TODO: Remove these once the GLib MessageLoopForUI isn't being used:
+// https://crbug.com/361635
+#define USE_GLIB 1
+#define USE_OZONE 1
+
+#endif  // defined(__ANDROID__)
+
 // A set of macros to use for platform detection.
 #if defined(__native_client__)
 // __native_client__ must be first, so that other OS_ defines are not set.
@@ -28,8 +62,7 @@
 #else
 #define OS_NACL_SFI
 #endif
-#elif defined(ANDROID)
-#define OS_ANDROID 1
+// Don't set OS_ANDROID; it's only used when building Chrome for Android.
 #elif defined(__APPLE__)
 // only include TargetConditions after testing ANDROID as some android builds
 // on mac don't have this header available and it's not needed unless the target