Remove assumption on absolute paths in java_cpp_enum.py.

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).

TBR=harryjin@google.com, jkellander@chromium.org
BUG=libyuv:536

Review URL: https://codereview.chromium.org/1581773003 .