DevTools: Fix CPU profile frames being offset by 1 sample.
DevTools: Fix CPU profile frames being offset by 1 sample.

In CPU profile samples delta for the first sample is an offset from
startTime.

BUG=847887

Change-Id: I200eae40770b521545c831f0e6e1ffa311cb7a85
Reviewed-on: https://chromium-review.googlesource.com/1079432
Reviewed-by: Aleksey Kozyatinskiy <562acbea96970f508ad9f5cc9fc7c62d82dd3ef8@chromium.org>
Commit-Queue: Alexei Filippov <52e51c43e283548b762e3c9dd9a4600768db9fb4@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#563006}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c7ca97d0c3786d65e81f25788ffcadbc302a8756
ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend,ChromeDevTools/devtools-frontend,dart-archive/devtools-frontend,dart-archive/devtools-frontend