IME: Improve performance of RenderFrameImpl::SyncSelectionIfRequired()

RenderFrameImpl needs a text around the selected text, and it was
obtained by WebLocalFrame::RangeAsText(). However, RangeAsText() is a
complex task, and WebInputMethodController has a pre-computed text. So
It's better getting the text through WebInputMethodController.

This CL should have no behavior changes. The new code path is guarded by
blink::features::kFastSelectionSync. It's in Blink because we're going
to add another Blink runtime flag depending on kFastSelectionSync.

Fixed: 414112253
Change-Id: Ib5f2be2da9d9202b172180edf841565eb72aba5e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6493967
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1453084}