Upgrade gRPC-C++ to 0.0.8, enabling experimental tvOS support (#2648)

Author: wilhuff

FirebaseFirestore.podspec

@@ -18,6 +18,7 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
 
   s.ios.deployment_target = '8.0'
   s.osx.deployment_target = '10.10'
+  s.tvos.deployment_target = '10.0'
 
   s.cocoapods_version = '>= 1.4.0'
   s.static_framework = true
@@ -51,13 +52,14 @@ Google Cloud Firestore is a NoSQL document database built for automatic scaling,
 
   s.dependency 'FirebaseAuthInterop', '~> 1.0'
   s.dependency 'FirebaseCore', '~> 5.2'
-  s.dependency 'gRPC-C++', '0.0.6'
+  s.dependency 'gRPC-C++', '0.0.8'
   s.dependency 'leveldb-library', '~> 1.20'
   s.dependency 'Protobuf', '~> 3.1'
   s.dependency 'nanopb', '~> 0.3.901'
 
   s.ios.frameworks = 'MobileCoreServices', 'SystemConfiguration'
   s.osx.frameworks = 'SystemConfiguration'
+  s.tvos.frameworks = 'SystemConfiguration'
 
   s.library = 'c++'
   s.pod_target_xcconfig = {