Refactoring: replace map with vector in FormPredictions.

This is pure refactoring, is replacing
using FormPredictions = std::map<uint32_t, PasswordFieldPrediction>;
with
using FormPredictions = std::vector<PasswordFieldPrediction>.

It simplifies code a little bit, and anyway the access is done with
range based loop.

The main point of this refactoring is that struct PasswordFieldPrediction
will be extended with names for iOS in the following CL.

Bug: 831123
Change-Id: I3cab46155ef4280ef0c413bfae740445f16f0ce1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1564159
Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Commit-Queue: Vadym Doroshenko <dvadym@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649897}