[remoting] Integrate with RemotingFtlMobileAPIKey

This CL implements GetRemotingFtlMobileAPIKey on google_apis and hooks
it up with FtlGrpcContext.

Bug: 954594
Change-Id: I8fb55de710f5ba5b0ee83127eada4c2b806ad8e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583001
Reviewed-by: Joe Downing <joedow@chromium.org>
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#655360}

google_apis/google_api_keys.h

@@ -76,6 +76,7 @@ std::string GetNonStableAPIKey();
 // Retrieves the Chrome Remote Desktop FTL API key to be used during the
 // signaling process.
 std::string GetRemotingFtlAPIKey();
+std::string GetRemotingFtlMobileAPIKey();
 
 #if defined(OS_IOS)
 // Sets the API key. This should be called as early as possible before this