linux: unexport as many symbols as possible

linux: unexport as many symbols as possible

See discussion on http://code.google.com/p/chromium/issues/detail?id=17557
On linux, exported symbols from a shared library can be overridden by another unit (e.g. the browser) if it exports the same ones. So compile everything with -fvisibility=hidden to make every symbol private by default, and explicitly export the ones that we care about. Also make sure that third-party deps (e.g. icu) don't explicitly export symbols.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=21992

[Greg Spencer, 2009-07-29 16:36:22]

LGTM