[LayoutNG] Stop asserting position validity in ComputeNGCaretPosition

The assertion was added when ComputeNGCaretPosition had a very limited
usage before. Now that it can be entered more often, and can be called
even on legacy layout (from ComputeContainingLineBoxOf(), which returns
null on legacy), the assertion no longer makes sense.

Hence, this patch removes the assertion.

Bug: 965518
Change-Id: Ibf64b23b5249c0af1935c12606527335d5f8101f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1622867
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661919}
1 file changed