Flag was added in https://chromium-review.googlesource.com/645408, over to author. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/523efae2ae9590d0bc4c0229277020db49fb5438

commit 523efae2ae9590d0bc4c0229277020db49fb5438
Author: Hitoshi Yoshida <peria@chromium.org>
Date: Thu Dec 13 03:24:03 2018

Field Trial: Drop runtime flag for V8ContextSnapshot

Experiment and field trial for V8 context snapshot project
was done successfully, and we no longer need this flag.
The feature is enabled by default on Windows/MacOS/Linux.

This CL is essentially a revert of https://chromium-review.googlesource.com/c/chromium/src/+/645408/


Bug:  912157 
Change-Id: I59d93bf3193ff4b5d81b1d9fb147b643f820568f
Reviewed-on: https://chromium-review.googlesource.com/c/1367066
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616192}
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/chrome/browser/about_flags.cc
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/chrome/browser/flag_descriptions.cc
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/chrome/browser/flag_descriptions.h
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/content/child/runtime_features.cc
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/content/public/common/content_features.cc
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/content/public/common/content_features.h
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/testing/variations/fieldtrial_testing_config.json
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/third_party/blink/public/platform/web_runtime_features.h
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/third_party/blink/renderer/platform/exported/web_runtime_features.cc
[modify] https://crrev.com/523efae2ae9590d0bc4c0229277020db49fb5438/tools/metrics/histograms/enums.xml