Corrects action source enum in docked windows histogram
Corrects action source enum in docked windows histogram

BUG=NONE
TEST=open UMA for Ash.Dock.ActionSource.
     DockedActionSource column should include "Keyboard"

Review URL: https://codereview.chromium.org/1424353002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#357230}

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