Revert "Change default VC++ compiler from 2015 to 2017"

This reverts commit ec9a1e6ce2708f93ea08f82dd76a1f556310af91.

Reason for revert: Weekend testing of VC++ 2017 is complete. A canary
was produced (62.0.3197.0) and a bug was found (crbug.com/759402), so
it was a worthwhile experiment.

Original change's description:
> 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-Commit-Position: refs/heads/master@{#497599}

TBR=thakis@chromium.org,brucedawson@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 683729, 727447
Change-Id: I9bef79f3197597fea6009692ec96809ff7ecf560
Reviewed-on: https://chromium-review.googlesource.com/637046
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#497684}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 43b6c849b3c49f71261a71fcb40455d0e263c49d

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 MSVS2017 as the default toolchain.
-CURRENT_DEFAULT_TOOLCHAIN_VERSION = '2017'
+# Use MSVS2015 as the default toolchain.
+CURRENT_DEFAULT_TOOLCHAIN_VERSION = '2015'
 
 
 def SetEnvironmentAndGetRuntimeDllDirs():