The root_build_dir variable yields the root of the build tree.
But when the component is being built under a non-default
toolchain, its output files are delivered to a toolchain
subdirectory instead.  The copy_compiler_dll rule should put its
output next to where libANGLE and things built using it will go,
which is root_out_dir.  This fixes cases where the angle GN
component is instantiated under more than one toolchain, which
heretofore would produce multiple Ninja rules for the same target
file $root_build_dir/d3dcompiler_47.dll and thus get complaints
from Ninja.

BUG=512869

Change-Id: I9fe83c5c5fa74902ef270654d3707636588af6b7
Reviewed-on: https://chromium-review.googlesource.com/320119Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Roland McGrath <mcgrathr@chromium.org>