Ensure text control entrance when generating clipboard text

Blink uses TextIteratorInFlatTree for generating clipboard text, which
used to enter text controls by default. The TextIterator behavior was
changed crrev.com/c/1327962, causing an unexpected behavior change.

This patch explicitly sets the EntersTextControls flag when computing
clipboard text to recover the original behavior.

Bug: 927394
Change-Id: I1e8d30cc73e14d5695b985b2e5afa1591e019663
Reviewed-on: https://chromium-review.googlesource.com/c/1450445
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628682}
2 files changed