fix(zone.js): __load_patch and __symbol__ should be in zone_extern for closure compiler

[JiaLiPassion]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: #30533
If we use Zone.__symbol__ in application or angular code, after closure compiler, the __symbol__ will be gone, because this API is internal and not exist in extern declaration file zone_extern.js. We need to add those APIs (symbol and __load_patch) because they are also public APIs.

What is the new behavior?

__symbol__ and __load_patch becomes public APIs.

Does this PR introduce a breaking change?

• Yes
• No

#30533 need this PR to work.

[mhevery]

presubmit

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