Build mac and linux release builds with symbols and don't strip them by default

On Mac, we already build with symbols, but strip them.

Building with symbols (a) matches what we do for official builds and (b)
makes crashes on release builders more useful.

BUG=315598
R=mark@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/66673005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234257 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234257