linux: enable PIE (ASLR) for chrome executable

I had been concerned that this wouldn't work with our -fPIC
bits we build internally, but the difference is actually a
performance once, not a correctness one.

BUG=57908

Review URL: http://codereview.chromium.org/6973032

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@85286 0039d316-1c4b-4281-b951-d872f2087c98