Fix NaCl IRT copy in GN chrome build.

Fix NaCl IRT copy in GN chrome build.

When we build the IRT in GN, it gets built into
out/*/irt_x64/nacl_irt.nexe, but it needs to be copied
to out/*/nacl_irt_x86_64.nexe.

There were two places in the GN build that depended on
the IRT; the chrome dependency only depended on the toolchain-specific
location, and the browser_tests dependency only worked if
the nacl browsertests are enabled (which they currently aren't,
by default).

As a result, we may not get the IRT build into the right
location, and the 'nacl_integration' build step might fail (though
it might succeed if it happened to find an old binary lying around).

Fixing the chrome dependency should cause the nacl_integration
step to pass again.

TBR=jam@chromium.org, phosek@chromium.org, brettw@chromium.org
BUG=529988

Review URL: https://codereview.chromium.org/1338453002

Cr-Commit-Position: refs/heads/master@{#348226}