Change default of fBuiltinFMASupport back to false

Being conservative here is better - it's a feature that's only exposed
on newer GL and ES. This fixes WebGL.

Follow-up from https://skia-review.googlesource.com/c/skia/+/317204

Bug: skia:10745
Change-Id: I319fc6b9379a14c09b9ccce6da8bed2effe09821
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/318497
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
2 files changed