GDTCCTUploader: lazy initialize url session, use ephemeral session (#6870)

* GDTCCTUploader: lazy initialize url session, use ephemeral session

* Changelog

* typo

Author: maksymmalyhin

GoogleDataTransport/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased
 - Remove `GCC_TREAT_WARNINGS_AS_ERRORS` from the podspec.
+- Reduce pre-main startup time footprint. (#6855)
 
 # v8.0.0
 - Source restructuring to limit the public API surface.

GoogleDataTransport/GDTCCTLibrary/GDTCCTUploader.m

@@ -80,6 +80,8 @@ @interface GDTCCTUploader () <NSURLSessionDelegate>
 
 @implementation GDTCCTUploader
 
+@synthesize uploaderSession = _uploaderSession;
+
 + (void)load {
   GDTCCTUploader *uploader = [GDTCCTUploader sharedInstance];
   [[GDTCORRegistrar sharedInstance] registerUploader:uploader target:kGDTCORTargetCCT];
@@ -101,12 +103,18 @@ - (instancetype)init {
   self = [super init];
   if (self) {
     _uploaderQueue = dispatch_queue_create("com.google.GDTCCTUploader", DISPATCH_QUEUE_SERIAL);
-    NSURLSessionConfiguration *config = [NSURLSessionConfiguration defaultSessionConfiguration];
+  }
+  return self;
+}
+
+- (NSURLSession *)uploaderSession {
+  if (_uploaderSession == nil) {
+    NSURLSessionConfiguration *config = [NSURLSessionConfiguration ephemeralSessionConfiguration];
     _uploaderSession = [NSURLSession sessionWithConfiguration:config
                                                      delegate:self
                                                 delegateQueue:nil];
   }
-  return self;
+  return _uploaderSession;
 }
 
 /**