Print a nicer error if installing an APK fails when you have not run adb root.

build/android/gyp/apk_install.py

Index: build/android/gyp/apk_install.py
diff --git a/build/android/gyp/apk_install.py b/build/android/gyp/apk_install.py
index 9431b29df0bbad750423bdb89bb0ada6fd3bb07f..ef10c6a3bd695403c8d661460f8e3a38d5ee930e 100755
--- a/build/android/gyp/apk_install.py
+++ b/build/android/gyp/apk_install.py
@@ -49,7 +49,10 @@ def RecordInstallMetadata(apk_package, metadata_path):
   """Records the metadata from the device for apk_package."""
   metadata = GetMetadata(apk_package)
   if not metadata:
-    raise 'APK install failed unexpectedly.'
+    if not android_commands.AndroidCommands().IsRootEnabled():
+      raise Exception('APK install failed unexpectedly -- root not enabled on '
+                      'the device (run adb root).')
+    raise Exception('APK install failed unexpectedly.')
 
   with open(metadata_path, 'w') as outfile:
     outfile.write(metadata)