pdf: Annotation pen options shouldn't open until selected

Adds a new attribute, 'open-after-select', to <viewer-toolbar-dropdown>. This
requires that 'selected' is first set before the dropdown will open.

Bug: 902646
Change-Id: I79f73b1b159f500680de24fb43cf10e74b0f911e
Reviewed-on: https://chromium-review.googlesource.com/c/1425838
Auto-Submit: dstockwell <dstockwell@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624778}
2 files changed