Introduce Selection#compute{Left,Top} in assert_selection()

This patch Introduces |selection#compute{Left,Top}| in |assert_selection()|
to provide left and top position for mouse click testing.

This patch is a preparation of the patch[1].

[1] http://crrev.com/2966473002: Convert editing/selection/shift-click.html to use
assert_selection()

BUG=679977
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2963113002
Cr-Commit-Position: refs/heads/master@{#483929}
2 files changed