Autofill: Improve performance of FieldDataManager::UpdateFieldDataMap()

It's called every time on updating text control values.

It was unable to move the specified value because it was
std::u16string_view. By changing it to `std::u16string`, the function
can move the value if a caller specifies a r-value as the argument.

This change improves typing delay by 1.3ms for a value including 3.3M
characters.

Bug: 341564372, 414858401
Change-Id: I7f39df1ccac6caadbd1f4675513fe85a1c15fb22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6555303
Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1462541}