Microdumps: refactor out common parts of minidump_writer.cc

This change is a pure refactoring of the common bits of minidump_writer.cc
that will be shared soon with the upcoming microdump_writer.cc.
In particular, this CL is extracting the following classes:
- ThreadInfo: handles the state of the threads in the crashing process.
- RawContextCPU: typedef for arch-specific CPU context structure.
- UContextReader: Fills out a dump RawContextCPU structure from the
  ucontext struct provided by the kernel (arch-dependent).
- SeccompUnwinder: cleans out the stack frames of the Seccomp sandbox
  on the supported architectures.
- MappingInfo: handles information about mappings

BUG=chromium:410294
R=mmandlis@chromium.org

Review URL: https://breakpad.appspot.com/4684002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1388 4c0a9323-5329-0410-9bdc-e9ce6186880e