Clean forward-declaration of Objective-C classes.
Clean forward-declaration of Objective-C classes.

Remove hack to support forward-declaring Objective-C classes as C++
classes since those headers are only included from Objective-C++
files.

BUG=298181

Review URL: https://codereview.chromium.org/1703493002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#375604}

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