Don't create redundant page text copies, replace with RefCountedString16

Feed the PageCaptured(...) consumers with a ref-counted string of the
page text instead of cloning.

Change-Id: Idc4eabe3ec38e7d0d8953d64f9a2fa0987993cf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5491252
Auto-Submit: Zinovy Nis <zynis@yandex-team.ru>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Anthony Cui <cuianthony@chromium.org>
Reviewed-by: thefrog <thefrog@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Basia Zimirska <basiaz@google.com>
Reviewed-by: Basia Zimirska <basiaz@google.com>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1294920}