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Add an ExtensionWantsToAct() method on ExtensionActionAPI.
Even though we're not quite sure what we want the UI to look
like for an extension desiring to act, we can start a little
bit of the plumbing now. For instance, exposing a method that
allows a caller to check if an extension wants to act on a
given page.

BUG=417441

Review URL: https://codereview.chromium.org/597413003

Cr-Commit-Position: refs/heads/master@{#297309}