Changes to support RBE in Windows builds (#820)

This PR does a few things:
1. It changes how the Windows toolchain and SDK are fetched and where
they are stored. Previously, the Windows toolchain and SDK were
downloaded under `depot_tools`, which typically exists outside of the
engine source root both locally and in CI. That setup makes the Windows
SDK inaccessible to RBE builds. Instead, this PR checks out
`depot_tools` under `flutter/third_party/depot_tools` by listing it in
the `DEPS` file. This location is under the engine source root, and is
therefore accessible to RBE builds. This change has the downside of
breaking how CI does caching of the `depot_tools` Windows toolchain and
SDK, which will add some time to builds.
2. It updates Windows GN toolchain setup logic to match what is in the
Chrome tree. Updates to `build/find_depot_tools.py`,
`build/toolchain/win/BUILD.gn`, and
`build/toolchain/win/setup_toolchain.py` are copied from there and
adapted to the Flutter engine repo.