Handle invisible ::first-letter in MostForwardCaretPosition

MostForwardCaretPosition asserts that, if we have moved to a visible
text node different from start node, the caret position can always be
found from the beginning of the text node, which is incorrect if the
text node contains invisible first letter part.

This patch fixes the incorrect assertion as the function already
returns correct result in this case.

Bug: 858965
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I8daa0ef59e06d4e9878c2e2c29bd12bf1825ead0
Reviewed-on: https://chromium-review.googlesource.com/1136944
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575563}
2 files changed