Fix GetMainModule() to properly get first module

Fix GetMainModule() to properly get first module

Change MinidumpModuleList::GetMainModule() to use GetModuleAtIndex() instead
of GetModuleAtSequence() because the former gets the first module that was
in the minidump file, while the latter actually gets the first module when
sorted by address.  While this is pretty much the same thing at the moment,
I have another change in the works that can sometimes affect the module
order in the file.

BUG=chromium-os:25355
TEST=Ran Breakpad tests
Committed: https://code.google.com/p/google-breakpad/source/detail?r=941

[Michael Krebs]

This is the upstream CL for https://gerrit.chromium.org/gerrit/18909

[Mark Mentovai]

LGTM