bug #24906 [Bridge/ProxyManager] Remove direct reference to value holder property (nicolas-grekas)

This PR was merged into the 2.7 branch.

Discussion
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[Bridge/ProxyManager] Remove direct reference to value holder property

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #23110
| License       | MIT
| Doc PR        | -

This fixes https://github.com/Ocramius/ProxyManager/issues/394 on Symfony's side, because proxy-manager v1 is not maintained anymore, but is the only one that works on PHP 5.3/5.5, which we still support.
Cannot be tested easily (but code is run by the existing test suite.)

Commits
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af9d6446 [Bridge/ProxyManager] Remove direct reference to value holder property