Currently, ProcessMemory is only implemented for Linux and Fuchsia.
Implement the interface for Windows as well and re-factor tests to
support it, mostly this consists of using a new ScopedGuardedPage class
instead of ScopedMmap in the ProcessMemory tests.
BUG=crashpad:262
Change-Id: I1b42718972be5ad838d12356d09f764053f09e4f
Reviewed-on: https://chromium-review.googlesource.com/c/1278829
Commit-Queue: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
