dump_syms: use unordered_set<> instead of set<> for speed.

dump_syms spends a lot of time trying to compare strings.
This change speeds up processing of libwebviewchromium.so by 30% on my linux
machine.

Patch by Junichi Uekawa <uekawa@chromium.org>

Review URL: https://breakpad.appspot.com/2714002/


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1341 4c0a9323-5329-0410-9bdc-e9ce6186880e