[chrome:omnibox]: create OmniboxInputs and OmniboxOutput custom HTML elements to encapsulate interaction with the DOM.

This CL is the first step to refactoring JS<->HTML communications. Actually refactoring output generation will be in a separate CL. More specifically, OmniboxOutput is a shell filled with <p> elements generated from the JS directly via document.createElement. In the upcoming CL's, this will be changed so that OmniboxOutput consists of custom elements as well.

Bug: 891303
Change-Id: I7b72ce3817aa7ad345f7c1ff7919668fd51aa4f0
Reviewed-on: https://chromium-review.googlesource.com/c/1257546
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598498}
3 files changed