[chromecast] Do not link with -latomic

"-latomic" flag is not necessary.

Besides, libatomic.so is unavailable in the next toolchain update.

BUG= internal b/33944287
TEST= builds.

Review-Url: https://codereview.chromium.org/2611173003
Cr-Commit-Position: refs/heads/master@{#442080}

base/BUILD.gn

@@ -1092,7 +1092,7 @@ component("base") {
   ]
 
   # Needed for <atomic> if using newer C++ library than sysroot
-  if (!use_sysroot && (is_android || is_linux)) {
+  if (!use_sysroot && (is_android || (is_linux && !is_chromecast))) {
     libs = [ "atomic" ]
   }