Linux: Pass --user-data-dir to the Zygote.
Linux: Pass --user-data-dir to the Zygote.

BUG=none
TEST=On Linux, "chrome --enable-logging --user-data-dir=/tmp/foobar" writes renderer logs to chrome_debug.log in /tmp/foobar rather than in $XDG_CONFIG_HOME.
Review URL: http://codereview.chromium.org/885003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@41369 0039d316-1c4b-4281-b951-d872f2087c98

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