Avoid raw pointers to traceable type in LineFlexer

Traceable types (like FlexItem) are allocated on the GCed heap and
can be reclaimed by the GC. Keeping a raw pointer to them off stack
(e.g. the backing store of Vector) may lead to use-after-free.

This CL avoid the raw pointers by keeping vectors of indices in
the original vector, rather than vectors of pointers.

Bug: 329102390
Change-Id: I823ebb2e9c0f6d1af1653b71d33e1466922d682f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6973558
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519223}