ninja/win: Let link-wrapper convert /s to \s in the linker command.

ExecLinkWrapper() runs a subprocess with shell=True, so that "link.exe" is
looked up using PATH from the environment loaded from environment.x86 in the
build directory.

shell=True implies that forward slashes in the path to the linker (say, if a
custom linker, such as lld-link.exe is used) don't work. If the linker command
is '..\../lld-link.exe', the command with fail with
"'..\..' is not recognized as an internal or external command".

So manually replace forward slashes with backslashes in the link command before
attempting to run it.

BUG=chromium:436277
Review URL: https://codereview.chromium.org/722073004/



git-svn-id: http://gyp.googlecode.com/svn/trunk@2009 78cadc50-ecff-11dd-a971-7dbc132099af