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After this change, DevToolsWindow waits for frontend to decide
on dock side, and only then shows it either docked or undocked.

This also cleans up DevToolsWindow public API, so clients don't use
test function, and test clients are explicit with their docking expectations.

Note: follow up patch on frontend side is require to switch
from SetDockSide to SetIsDocked call.

BUG=318751

Review URL: https://codereview.chromium.org/108213012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245308 0039d316-1c4b-4281-b951-d872f2087c98