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The dedicated folders do not add much value, and in-fact get in the way by
resulting in too many BUILD.gn files and js_type_check targets.

For example viewer-pen-options:closure_compile target was not referenced
anywhere (fixed).

Merging all elements to a single folder makes things a bit easier for the
Polymer2 to 3 migration.

Bug: 1005029
Change-Id: Ic01de6e17486417dee79de791baddf3d9b338f60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815078
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698645}