Add method to add/extract extensions to/from TaskTraits

Extension writers will have to implement the new
TaskTraitsExtensionDescriptor and keep it in sync with the corresponding
cpp code.

Bug: 863341
Change-Id: Ib55433feaf47616cb9c9b793f546a0e47e977111
Reviewed-on: https://chromium-review.googlesource.com/c/1461031
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631173}
4 files changed