Change default VC++ compiler from 2015 to 2017

Now that VC++ 2017 15.3.2 is packaged up we can try again to have VC++
2017 as the default compiler. This will be reverted at the end of the
weekend. If the clang workaround for 727447 is reverted then this will
also test whether that code-gen bug is fixed.

R=thakis@chromium.org
BUG=683729,727447

Change-Id: I74b070a0f8b5c58348309e1bea553136499c6922
Reviewed-on: https://chromium-review.googlesource.com/636325
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#497599}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ec9a1e6ce2708f93ea08f82dd76a1f556310af91

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():