Set an empty sourcepath when compiling java.

build/android/gyp/javac.py

Index: build/android/gyp/javac.py
diff --git a/build/android/gyp/javac.py b/build/android/gyp/javac.py
index 728c49d5eef513191a56f85993dbf368898ffbfe..deb90c5de64e03fb28c562146625da3865833df7 100755
--- a/build/android/gyp/javac.py
+++ b/build/android/gyp/javac.py
@@ -322,6 +322,9 @@ def main(argv):
       # javac pulling a default encoding from the user's environment.
       '-encoding', 'UTF-8',
       '-classpath', ':'.join(compile_classpath),
+      # Prevent compiler from compiling .java files not listed as inputs.
+      # See: http://blog.ltgt.net/most-build-tools-misuse-javac/
+      '-sourcepath', ''
   ))
 
   if options.bootclasspath: