minidump-2-core: Allow user to override shared lib basedir.

This CL introduces "--sobasedir" command-line option to minidump-2-core.
By default, paths to shared lib files included in generated core file are
formatted as "/var/lib/breakpad/[GUID]libFoobar.so". By using "--sobasedir",
this will be "[user-supplied-path]libFoobar.so".

This option can be used to ease debugging minidump files from developer's local
builds. By specifying "minidump-2-core --sobasedir `pwd`/out/Debug/lib/
foobar.mdmp", user can load the generated core file in gdb without setting up
the symlinks for "/var/lib/breakpad".

BUG=None
A=kouhei@chromium.org

Original code review: https://breakpad.appspot.com/1214002/

R=kouhei@chromium.org

Review URL: https://breakpad.appspot.com/1584002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1317 4c0a9323-5329-0410-9bdc-e9ce6186880e