Check IBUS_CONFIG_HOME environment variable when reading/writing the socket file.
Check IBUS_CONFIG_HOME environment variable when reading/writing the socket file.

BUG=crosbug.com/16501
TEST=export IBUS_CONFIG_HOME=/tmp, start Chrome on Chromium OS, verify an empty dir /tmp/ibus/bus exists with mode 0700, log in, enable IME, verify IME works, verify the socket file is created in /tmp/ibus/bus.

Review URL: http://codereview.appspot.com/4621043

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