User the assetpath for viewer-button in the PDF viewer toolbar

This uses the assetpath for viewer-button images so that the viewer-button element
can be uesd from any location in the codebase.

TBR=ganetsky
BUG=303491

Committed: a7e5476

Review URL: https://codereview.chromium.org/132333015
1 file changed
tree: ea998bb4231cd26ec25925ed1f87639ba74ea7f9
  1. elements/
  2. codereview.settings
  3. OWNERS