Suppress build warnings

[chuganzy]

This PR is based on https://github.com/hisaichi5518/native_webview/pull/109, so will make this ready after merging it.

Commits:

• https://github.com/hisaichi5518/native_webview/commit/445a3aaa17709bd722d6b2a0dc34b0889de3d85e
• https://github.com/hisaichi5518/native_webview/pull/110/commits/c28571db593466a287fb17f4f18a52400d545207