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To support dynamically added/removed content scripts in <webview>, we store
these content scripts in the same shared memory where user scripts from
declarative content API are stored, but the decision for injection will
be made in the render once the url pattern is matched. Since render doesn't
need to send IPCs to ask a decision from browser, it makes a precise time
injection possible.

Special to <webview>, once the content scripts are updated (added/removed),
browser will only notify the render process of the given <webview>, rather than
all of the renders (which is the case for user scripts).

BUG=437566

Review URL: https://codereview.chromium.org/906493004

Cr-Commit-Position: refs/heads/master@{#318774}