The VS 2015 tools require the Windows 10 Universal C Runtime, either
installed in C:\Windows or in every directory containing tools, or
dynamically linked executables created with VS 2015. Installing to
C:\Windows seems less error prone.

This is only applicable for Google developers and build machines that
don't have VS 2015 installed.

This updates the packaging script so that it packages the three
installers, and no longer packages the installed files (which vary
between operating systems anyway).

The installer is updated to check for the existence of one of the
Universal C Runtime files. If it isn't found then it detects the
version of Windows in order to select and run the correct installer.

I manually confirmed that, for instance, the installers for Windows 7
and Windows 2008 R2, were identical despite coming from different
download URLs.

If the installation fails because gclient runhooks is run non-elevated
then the developer will have to do a one-time manual install of the
update. A message will be printed indicating this.

BUG=440500

Review URL: https://codereview.chromium.org/1588673004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298286 0039d316-1c4b-4281-b951-d872f2087c98