Rename fDeclared to fVarExists.

We can't eliminate the field, but we can make it more obvious what it
tracks.

Change-Id: Ia2eb558d869cdf7911c591d0b66da89dd322384b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/583960
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>