Don't bind a null LayoutText pointer to a reference

https://chromium-review.googlesource.com/c/chromium/src/+/5308656
introduced a null reference, which is undefined behavior and flagged by
UBSan. If text_layout_object was null, it is UB to write
*text_layout_object. Although the parameter was ignored
(ShouldUpdateLayoutByReattaching will return true and thus
UpdateTextLayoutObject will ignore layout_text), even binding the
reference is too late.

We could fix this by making LayoutText a pointer, but then that method's
API contract is weird because layout_text would need to be non-null when
by_reattach is true. Instead, since this helper method is so short and
only called twice, I've simply inlined it into the two call sites.

Bug: 40248746
Change-Id: I665a509f89c9351a4f59eeaa980bf5289e250cb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5383408
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1276741}