Learned that cancelled is appropriate for non-US English, but the function is
IsCanceled, so I went with consistency.
Change-Id: If63a8af75338780c2f8e81bf4404fc4b5f852d77
Instead, require specifying it when constructing a new context.
Prevents misuse and bugs / data races.
Change-Id: I775e998bf69b9e238ec726c469494c1a65f8f9de
Will eventually be plumbed through lots of APIs, especially those requiring or benefiting from
cancelation notification and/or those needing access to the HTTP context (e.g. App Engine).
Change-Id: I591496725d620126e09d49eb07cade7707c7fc64
Bill Thiede