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}

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 = '2015'
+CURRENT_DEFAULT_TOOLCHAIN_VERSION = '2017'
 
 
 def SetEnvironmentAndGetRuntimeDllDirs():