On Linux, Perfetto translates the builtin "ts" timestamp in trace event
from CLOCK_MONOTONIC to CLOCK_BOOTTIME, before passing them to devtools.
Devtools therefore implicitly operates on timestamps that are in
CLOCK_BOOTTIME.

However, additional timestamps sent in trace event payload arguments
will not be converted to CLOCK_BOOTTIME by Perfetto, raising the
possibility of devtools using timestamps from multiple clock domains
incorrectly.

Since trace events sent by CpuProfile also include the builtin "ts"
trace timestamp (sampled from CLOCK_MONOTONIC nearly at the same time by
the tracing framework), sending "data.startTime" and "data.endTime" is
essentially redundant. devtools-frontend:2113957 stops the use of the
value of these timestamps in the payload of Profile and ProfileChunk
events. Devtools continue to use the presence of these arguments to
indentify start and end profile events.

ProfileChunk events also include "timeDeltas" which are relative
timestamps. They are also in CLOCK_MONOTONIC and are not translated by
Perfetto. devtools-frontend:2113957 computes absolute CLOCK_BOOTTIME
timestamps from timeDeltas by adding them to "ts" in the "Profile" event
(previously, "data.startTime" was used). This is only valid if the
system is not suspended/resumed during profiling. Providing support for
suspend/resume in the middle of profiling will likely involve having
Perfetto convert "timeDeltas" directly to CLOCK_BOOTTIME.

This CL introduces no code changes and only adds comments to explain
the above.

BUG=chromium:1055871

Change-Id: I649dfcce8ea1a100c0ecfe03f843c7cb1fdd6f33
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2114001
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66820}