Add scripts to upload / to download prebuilt mojo_shell

This adds some scripts that download prebuilt mojo_shell binaries
from google storage (linux only for now) and gyp/gn flags to toggle
using these prebuilt binaries instead of ones built from source. This
also adds a helper script that uploads a locally built mojo_shell binary
to the appropriate place in google storage.  The versioning scheme is:

*) mojo/public/VERSION contains an identifier (committish currently) for
  the mojo public SDK. The mojo_shell binary must match the interfaces
  defined in this version of mojo/public/.

*) gs://mojo/shell/$VERSION/$PLATFORM.zip contains a prebuilt mojo_shell
  binary for a particular version. $PLATFORM for now must be linux-x64,
  but we can add more later as needed.

gclient runhooks downloads the shell binary referred to by
mojo/public/VERSION if it is available for the host platform to
mojo/public/tools/prebuilt/ and updates mojo/public/tools/prebuilt/VERSION
to tag the version downloaded if successful. On subsequent runs the hook
will check this against mojo/public/VERSION and avoid downloading again if
the binary is up to date.

This also adds variables to the GYP and GN builds that control if building
'mojo' builds a mojo_shell binary from source or copies the prebuilt binary
into the outdir.  They both default to false for now.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/671983003