Skip ElfCoreDumpTest and LinuxCoreDumperTest on Android if no core file is dumped.

On certain versions of Android (specifically JellyBean MR2 on Nexus 7, possibly
others too) no ELF core dump is created for crashing processes.  Check for this
and skip the test if so.

BUG=364943
R=thestig@chromium.org

Review URL: https://breakpad.appspot.com/1624003

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1318 4c0a9323-5329-0410-9bdc-e9ce6186880e