Introduce SelectedMap class to LayoutSelection::SetSelection()

LayoutSelection::SetSelection defines 2 inner classes and 2 variables for that.
This CL defines the new class to combine them.

TEST=No change in behavior.
BUG=708453

Review-Url: https://codereview.chromium.org/2873953002
Cr-Commit-Position: refs/heads/master@{#470839}
1 file changed