Add StreamStartAlgorithm type

StartAlgorithms are different from other promise-returning algorithms in
the streams standard in that they can throw an exception. Add a new
StreamStartAlgorithm base class to represent them, and a
CreateStartAlgorithm() function to create ones from JavaScript.

Design doc:
https://docs.google.com/document/d/1n0IIRmJb0R-DFc2IhhJfS2-LUwl6iKSBNaR0klr3o40/edit

BUG=902633

Change-Id: I0995aa21bbec51ab206a4affc54d861b2c325db4
Reviewed-on: https://chromium-review.googlesource.com/c/1445342
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#627877}
4 files changed