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This CL introduce DeclarativeUserScriptManager to manager a set of DeclarativeUserScriptMaster objects which were stored in the ExtensionSystemImpl.

With this refactor, it allows the DeclarativeUserScriptManager to take care of the creations of DeclarativeUserScriptMaster objects:
1) It enable the DeclarativeUserScriptManager accept all kinds of input keys and convert them as a uniform format;
2) The manager will assign a unique ID to the consumer who requests a DeclarativeUserScriptMaster object.The consumer can use this ID to do script injection.
(The above two steps will be implemented in another CLs)

This refactor is a part of issue "support DeclarativeContent script injection on <webview>", please refer to a design doc(https://goto.google.com/introduceidforscriptinjection).
BUG=437566

Review URL: https://codereview.chromium.org/796453003

Cr-Commit-Position: refs/heads/master@{#308171}