linux: use /proc/self/exe when exec'ing ourselves
linux: use /proc/self/exe when exec'ing ourselves

We don't want to ever go out to disk when looking for data after
startup, since they can be changed by an update.

We *should* just be using the zygote, but the zygote is sandboxed-only
for now, and fixing that has a lengthy dependency graph.  In the
interim, /proc/self/exe is the correct executable and exec'ing it should
work as long as the subprocess doesn't need any other files from the
Chrome directory.

BUG=22703
TEST=Start Chrome.  Move away the entire Chrome directory; plugins
should still work.

Review URL: http://codereview.chromium.org/403018

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@32410 0039d316-1c4b-4281-b951-d872f2087c98

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