innerText setter should not modify an existing Text child

We have an optimization reusing a Text child, but the standard doesn't
define such behavior, and Edge, Firefox, and Safari don't have it.

This CL might regress benchmarks.  We'll accept it.

Bug: 573309
Change-Id: I3aacae5d37e5a7685ce2d310944dfef8d6e4b13a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1502338
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637665}
13 files changed