storage: increase default chunk size to 16MB

Based on research by bjornleffler@ and domz@, this chunk size is
the best default for performance.

Fixes #437

Change-Id: I7c61549d2bd71d533b1512cba523a93afe1e97b8
Reviewed-on: https://code-review.googlesource.com/c/google-api-go-client/+/50290
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Cody Oss <codyoss@google.com>
Reviewed-by: Frank Natividad <franknatividad@google.com>
diff --git a/googleapi/googleapi.go b/googleapi/googleapi.go
index 4431716..471ae25 100644
--- a/googleapi/googleapi.go
+++ b/googleapi/googleapi.go
@@ -54,7 +54,7 @@
 
 	// DefaultUploadChunkSize is the default chunk size to use for resumable
 	// uploads if not specified by the user.
-	DefaultUploadChunkSize = 8 * 1024 * 1024
+	DefaultUploadChunkSize = 16 * 1024 * 1024
 
 	// MinUploadChunkSize is the minimum chunk size that can be used for
 	// resumable uploads.  All user-specified chunk sizes must be multiple of