Add import from string to PasswordImporter

This CL adds a new function to PasswordImporter to be able to
import not only from a file, which is already the case, but also
from a string. The rest of the logic is identical.

This will be useful to directly stream password data extracted
to a string from a zip file without having to write that data
back into a file before importing it.

This CL also adds a related test to make sure importing from a
string works as well as importing from a file.

Design doc: go/bling-import

Bug: 407587751
Change-Id: I6c1910cd5abd17a0f2d4a812f61011884af5e24c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6548536
Reviewed-by: Andrii Natiahlyi <natiahlyi@google.com>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1462830}