Revert "Make VS 2017 the default compiler on Windows"

This reverts commit 8707de2.

Reason for revert: media_unittests is crashing in vpx_internal_error
https://build.chromium.org/p/chromium.win/builders/Win10%20Tests%20x64/builds/15967

Original change's description:
> Make VS 2017 the default compiler on Windows
> 
> All known blockers for VS 2017 as the default compiler for Chrome on
> Windows have been addressed. Switching to VS 2017 will give us:
> 
> - Better C++ conformance and various bug fixes, especially around
> constexpr
> - Faster fastlink links and fixes to linker bugs which can cause
> incremental linking to fail
> - Newer STL header files for increased STL conformance
> - Debugger stability fixes (it is quite possible to debug VS 2015
> binaries with VS 2017 but using the same toolchain for both is
> preferable)
> 
> Note that many of these benefits apply even in the context of switching
> to clang - faster links are always good, and increased VS conformance
> makings switching between VS and clang easier.
> 
> Test switches to VS 2017 have been done enough times to make us
> reasonably confident that the switch will go smoothly. If not then it
> can be easily reverted. If the switch sticks then the documentation
> pages will need to be updated.
> 
> Note that the Windows 10 Creators Update SDK will be made required soon
> after this change.
> 
> Bug: 683729
> Change-Id: I79fdd9e44c6bb7ef25ab6bcb88845ac18e5cf56f
> Reviewed-on: https://chromium-review.googlesource.com/665637
> Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Reviewed-by: Scott Graham <scottmg@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#502421}

TBR=dpranke@chromium.org,brucedawson@chromium.org,scottmg@chromium.org

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

Bug: 683729
Change-Id: Icf40a84f30941980b1c9e038cc4336b057ea10c0
Reviewed-on: https://chromium-review.googlesource.com/670283
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502535}

build/vs_toolchain.py

@@ -22,8 +22,8 @@
 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():