These assertions were helpful back when the client manually managed
offsets. Now that offsets are managed automatically by the toBuffer()
logic, they don't carry any benefit.
Removing these assertions will pave the way for allowing a summary to be
created, serialized, modified, and then serialized again, which we will
need to do to efficiently generate both "full" and "semantic" summaries.
R=brianwilkerson@google.com
Review URL: https://codereview.chromium.org/2110693002 .
Paul Berry
efe5a31421