[Backport] Remove assumption on absolute paths in java_cpp_enum.py.

The same issue in libyuv also affects us. Original commit message:

  In java_cpp_enum.py there were assumptions on 'build' being
  a component of the absolute path of the script invoking the
  script. This leads to problems when other projects are using
  the Chromium build toolchain (in this case libyuv), where
  the script invoking GYP is located in the root of the project
  dir (https://chromium.googlesource.com/libyuv/libyuv/+/master/gyp_libyuv).

  BUG=libyuv:536

  Review URL: https://codereview.chromium.org/1571243003

BUG=XWALK-5749

build/android/gyp/java_cpp_enum.py

@@ -229,10 +229,7 @@ def _ParseRegularLine(self, line):
       self._in_enum = True
 
 def GetScriptName():
-  script_components = os.path.abspath(sys.argv[0]).split(os.path.sep)
-  build_index = script_components.index('build')
-  return os.sep.join(script_components[build_index:])
-
+  return os.path.basename(os.path.abspath(sys.argv[0]))
 
 def DoGenerate(source_paths):
   for source_path in source_paths: