Expose BlobData::appendBytes() as there're multiple places where data is added from char*

Also change the type of the length argument of appendBytes() to size_t.
There's no need to use long long for it.

BUG=none

Review URL: https://codereview.chromium.org/562843002

git-svn-id: svn://svn.chromium.org/blink/trunk@182221 bbb929c8-8fbe-4397-9dbb-9b2b20218538