[HttpKernel] added $event->isMasterRequest()
merged branch kriswallsmith/kernel/is-master-request (PR #8702)

This PR was merged into the master branch.

Discussion
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[HttpKernel] added convenience method $event->isMasterRequest()

Prettier code and easier mocking.

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

Commits
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554f95f [HttpKernel] added $event->isMasterRequest()