[cr-action-menu] Use clientWidth for rtl flipping.

This CL fixes a bug where the action menu would use the entire body's
length as the viewport when flipping coordinates in RTL.

It also moves the scroll position saving into showAtPosition so that
both show methods account for scroll correctly.

BUG=734984
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2966163004
Cr-Commit-Position: refs/heads/master@{#485181}
2 files changed