net/http: reduce allocs in CrossOriginProtection.Check

[jub0bs]

Rather than repeatedly creating error values on
CrossOriginProtection.Check's unhappy paths, return non-exported and
effectively constant error variables.

For #73626.

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from jub0bs:

Patch Set 1:

(1 comment)

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Message from qiu laidongfeng2:

Patch Set 1: Code-Review+1 Commit-Queue+1

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Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-06-17T15:26:58Z","revision":"c8196b5bf674241264a2cdf9f21365840629b2b8"}

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Message from qiu laidongfeng2:

Patch Set 1: -Commit-Queue

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Patch Set 1:

This CL has passed the run

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Patch Set 1: LUCI-TryBot-Result+1

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Message from Sean Liao:

Patch Set 1: Auto-Submit+1 Code-Review+2

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Message from Dmitri Shuralyov:

Patch Set 1: Code-Review+1

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Patch Set 2: Code-Review+1

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This PR is being closed because golang.org/cl/681178 has been merged.