Chromecast: User-Agent string should have "/" before build number.

R=lcwu@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/616943002

Cr-Commit-Position: refs/heads/master@{#297454}

chromecast/shell/common/cast_content_client.cc

@@ -15,7 +15,7 @@ namespace shell {
 std::string GetUserAgent() {
   std::string product = "Chrome/" PRODUCT_VERSION;
   return content::BuildUserAgentFromProduct(product) +
-      " CrKey" CAST_BUILD_REVISION;
+      " CrKey/" CAST_BUILD_REVISION;
 }
 
 CastContentClient::~CastContentClient() {