Fix: Correct EventLatency for Synthetic Scroll Events

This patch fixes the EventLatency calculation for synthetic gesture
scroll update events generated by the ScrollPredictor.

Previously, the metrics for synthetic events were created using
`frame_time` as the timestamp. This was incorrect because the synthetic
event is actually generated and resampled at `sample_time` (frame_time +
kResampleLatency). This mismatch caused inaccurate EventLatency slices
and affected the JankV4 metric, as described in b:445641653.

This change updates the creation of `cc::ScrollUpdateEventMetrics`
within `ScrollPredictor::GenerateSyntheticScrollUpdate` to use
`gesture_event.TimeStamp()` (which corresponds to the `sample_time`) for
the `timestamp`, `arrived_in_browser_main_timestamp`, and
`blocking_touch_dispatched_to_renderer` fields. This ensures the latency
metrics accurately reflect the synthetic event's timing.

Bug: b:445641653, b:444669046
Change-Id: I3035cfc005597f6c43459da842dedbc9b1a63db5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965286
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Aman Verma <amanvr@google.com>
Cr-Commit-Position: refs/heads/main@{#1519442}