[Autofill Assistant] Introduce Selector struct to replace vectors.

Before this CLs, all methods handling selectors took a
std::vector<std::string> as input.

This CL replaces all vectors of strings by a Selector struct, such that
it will be easier to add parameters (like pseudo type) to a selector.

Change-Id: Ic981282db840b02e996c21ceb1072607ff25606e
Reviewed-on: https://chromium-review.googlesource.com/c/1348049
Commit-Queue: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Reviewed-by: Stephane Zermatten <szermatt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610550}
41 files changed