Test changing default Windows compiler to VS2017

Doing another VS2017 test over the week end now with the newly packaged
VS 2017 Update 3 Preview 4 (was Preview 2 last time).

This CL is currently purely for testing purposes and will be reverted by
the end of the weekend.

R=dpranke@chromium.org
BUG=683729

Change-Id: Ie5c2e92b789b1dcbd51d4e0028230b5b72ce9eb9
Reviewed-on: https://chromium-review.googlesource.com/572500Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486999}

build/vs_toolchain.py

@@ -22,8 +22,8 @@ sys.path.insert(0, os.path.join(chrome_src, 'tools', 'gyp', 'pylib'))
 json_data_file = os.path.join(script_dir, 'win_toolchain.json')
 
 
-# Use MSVS2015 as the default toolchain.
-CURRENT_DEFAULT_TOOLCHAIN_VERSION = '2015'
+# Use MSVS2017 as the default toolchain.
+CURRENT_DEFAULT_TOOLCHAIN_VERSION = '2017'
 
 
 def SetEnvironmentAndGetRuntimeDllDirs():