Opt-in burst mechanism for pipeline shaders

Add a dedicated virtual for shaders to select burst mode.

Enabled for linear gradients with more than 8 stops, as a start.

BUG=skia:6710

Change-Id: I1b19124f42d1d805de27a5db6a26601c386bb9ff
Reviewed-on: https://skia-review.googlesource.com/18628
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
5 files changed