Revert of Changing default Windows compiler to VS 2017 (patchset #1 id:1 of...

Revert of Changing default Windows compiler to VS 2017 (patchset #1 id:1 of https://codereview.chromium.org/2852433005/ )

Reason for revert:
Reverting as planned - it was just a test switch to VS 2017, which seems to have gone smoothly. I'm hopeful that canary 60.0.3086.0 is being built with VS 2017, to
complete the test.

Original issue's description:
> Changing default Windows compiler to VS 2017
>
> This is a single character change to temporarily switch Chrome to
> building with VC++ 2017. This CL is currently purely for testing
> purposes and will be landed for the weekend in order to flesh out any
> remaining bugs.
>
> This is a retry of crrev.com/2762093003 which failed due to a compiler
> warning which was missed in local testing and the try bots. That warning
> has been resolved and a local build of 'all' completed cleanly.
>
> BUG=683729
>
> Review-Url: https://codereview.chromium.org/2852433005
> Cr-Commit-Position: refs/heads/master@{#468239}
> Committed: https://chromium.googlesource.com/chromium/src/+/060d293f1d87d450a824dc46f564792272ed34a3

TBR=thakis@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=683729

Review-Url: https://codereview.chromium.org/2851143002
Cr-Commit-Position: refs/heads/master@{#468298}

build/vs_toolchain.py

@@ -22,7 +22,7 @@ json_data_file = os.path.join(script_dir, 'win_toolchain.json')
 
 
 # Use MSVS2015 as the default toolchain.
-CURRENT_DEFAULT_TOOLCHAIN_VERSION = '2017'
+CURRENT_DEFAULT_TOOLCHAIN_VERSION = '2015'
 
 
 def SetEnvironmentAndGetRuntimeDllDirs():