re-re-land of skslc now automatically turns on derivatives support

Only change from last attempt is putting the call to shaderDerivativeExtensionString behind a check for shaderDerivativeSupport to avoid a spurious assertion failure.

TBR=benjaminwagner@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2437063002

Review-Url: https://chromiumcodereview.appspot.com/2437063002
18 files changed