Allow Win64 build of base library even in a Win32 build.

Some components of a 32 bit build of chrome for Windows need to be built
as Win64 Dlls.  To allow those components to use base, we make a Win64
build of the base library even when building chrome 32 bit.

This is needed by the Chrome Desk Band code. crbug.com/327435.
https://codereview.chromium.org/79173004/

BUG=327435

Review URL: https://codereview.chromium.org/103333004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243936 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243936