[ios] Fixed -[CRWWebController isCurrentNavigationItemPOST].
[ios] Fixed -[CRWWebController isCurrentNavigationItemPOST].

Use HTTP Method instead of navigation type to determine the result.

BUG=None

Review-Url: https://codereview.chromium.org/1966063002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#393013}

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