[ScrollTimeline] Upstream based currentTime tests to WPT

This CL starts to upstream the currentTime tests to WPT. To split this
up, we start with just the 'basic' tests - basic functionality,
adjusting for time range, and start/end scroll offsets. The writing mode
and NaN tests will follow in later patches.

There are a few mostly non-behavioral changes to the tests in this CL:

  * Some comments were edited for clarity.
  * For tests with more than one assert_equals(), descriptions were
    added to the asserts. This is to help locate failing assert_equal
    lines, as jsharness does not print out the line number of a failure.
  * In a few places we had more than one assert_equals for the same
    scenario (e.g. multiple cases where scroll was after the
    startScrollOffset point). These have been deduplicated.

Bug: 911254
Change-Id: I783fcf7f43cda876defa9c36db9a0e7dff4e82c7
Reviewed-on: https://chromium-review.googlesource.com/c/1366297
Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614510}
3 files changed