Move blob-related code to platform/blob

This CL breaks a dependency from core/platform/network on core/fileapi but
moving some blob-related code into platform/blob. The DOM APIs for blobs are
still in core/fileapi, but the code that supports BlobData is now in the
platform layer and has proper dependencies.

BUG=297477

Review URL: https://codereview.chromium.org/26283005

git-svn-id: svn://svn.chromium.org/blink/trunk@159918 bbb929c8-8fbe-4397-9dbb-9b2b20218538