Add support for viewtimeline startOffset and endOffset

Bug: 1385116
Change-Id: Iea32072290b355e843f104c3b8d15aea01a69b56
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4029457


Commit-Queue: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1072416}