This reverts commit d84b4664.

Reason for revert: Fails "Mutable Constants" check on android-binary-size: https://ci.chromium.org/ui/p/chromium/builders/try/android-binary-size/1211670/overview

For details about this check, see https://chromium.googlesource.com/chromium/src/+/main/docs/speed/binary_size/android_binary_size_trybot.md#Mutable-Constants

Original change's description:
> [flags] Rename v8_flags to FLAGS
>
> Team members expressed concerns that "v8_flags" is easier to miss in the
> code than the previous "FLAG_" syntax. After a poll and discussions we
> decided to rename the struct to "FLAGS", so the new syntax for
> addressing flag values is "FLAGS.foo" instead of the previous
> "FLAG_foo".
>
> R=​cbruni@chromium.org
> CC=​jkummerow@chromium.org
>
> Bug: v8:12887
> Change-Id: I51af4aa7fd5a3b3c29310c0cb4c4ff42086ff012
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3854508
> Commit-Queue: Clemens Backes <clemensb@chromium.org>
> Reviewed-by: Camillo Bruni <cbruni@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#82701}

Bug: v8:12887
Change-Id: I75516a0be9bc475afa2bbaa96a05e8a9b5be9be7
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3855936
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82711}