app/appengine: explicitly mark the Log.CompressedLog field as not indexed
app/appengine: explicitly mark the Log.CompressedLog field as not indexed

The old App Engine datastore API and the new Cloud datastore API have
different behaviors with regard to indexing []byte properties.

Both APIs say the same:

https://godoc.org/google.golang.org/appengine/datastore#hdr-Properties
https://godoc.org/cloud.google.com/go/datastore#hdr-Properties

> All fields are indexed by default. Strings or byte slices longer
> than 1500 bytes cannot be indexed; fields used to store long strings
> and byte slices must be tagged with "noindex" or they will cause Put
> operations to fail.

But empirically only the new cloud API actually returns an error when
putting a >1500 []byte value into a field without noindex. Not sure
why.

Updates golang/go#34744

Change-Id: I15c938e91c92304eb7074de99c19f8e93182283e
Reviewed-on: https://go-review.googlesource.com/c/build/+/210998
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/build,golang/build,golang/build,golang/build