Do not try to link chrome or app_shell in chromeos gn build.

On ChromeOS, Chrome links in the remoting host, which isn't gn-ified yet.
I'm not quite sure why app_shell doesn't work, but it might be the same
issue.

TBR=cmasone@chromium.org
BUG=424337

Review URL: https://codereview.chromium.org/697223004

Cr-Original-Commit-Position: refs/heads/master@{#302524}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6fc75b3f8718593090cca2b4a6cba0e84122d201

config/BUILDCONFIG.gn

@@ -32,7 +32,7 @@ declare_args() {
 
   # Set to true when compiling with the Clang compiler. Typically this is used
   # to configure warnings.
-  is_clang = (os == "mac" || os == "ios" || os == "linux")
+  is_clang = (os == "mac" || os == "ios" || os == "linux" || os == "chromeos")
 
   # Forces a 64-bit build on Windows. Does nothing on other platforms. Normally
   # we build 32-bit on Windows regardless of the current host OS bit depth.