Support MSVC depot tools toolchains for 3rd parties

Third parties would construct toolchain packages on their own,
with the best of their abilities, based on the instructions
provided.

They could then accumulate <canonical hash> -> <their own hash> pairs
in their environment to support historical builds.

Example mapping:
set GYP_MSVS_HASH_1180cb75833ea365097e279efb2d5d7a42dee4b0=4181fce548fece344a85cb6898cf996692998531

Bug: 836225
Change-Id: I4a46e51b09049340344c9717481b141a2a7c296b
Reviewed-on: https://chromium-review.googlesource.com/1025896Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Kimmo Kinnunen FI <kkinnunen@nvidia.com>
Cr-Commit-Position: refs/heads/master@{#553476}

build/vs_toolchain.py

@@ -346,7 +346,11 @@ def _GetDesiredVsToolchainHashes():
   if env_version == '2017':
     # VS 2017 Update 3.2 with 10.0.15063.468 SDK, patched setenv.cmd, and
     # 10.0.16299.15 debuggers.
-    return ['1180cb75833ea365097e279efb2d5d7a42dee4b0']
+    toolchain_hash = '1180cb75833ea365097e279efb2d5d7a42dee4b0'
+    # Third parties that do not have access to the canonical toolchain can map
+    # canonical toolchain version to their own toolchain versions.
+    toolchain_hash_mapping_key = 'GYP_MSVS_HASH_%s' % toolchain_hash
+    return [os.environ.get(toolchain_hash_mapping_key, toolchain_hash)]
   raise Exception('Unsupported VS version %s' % env_version)