Use stricter shadowing warning flags in Flutter targets.

[chinmaygarde]

No description provided.

[chinmaygarde]

This is related to the comment @stuartmorgan made on another commit about shadowing in lambda captures. I incorrectly stated that this would have been caught with the default shadowing warning flags. This is not true apparently and all shadowing flags are only enabled via the -Wshadow-all flag. We cannot yet enable the same because Skia headers don't fully compile with the stricter warnings. But I will patch them one by one and enable the strictest set of flags. We can also use the current spot to enable Flutter specific warnings in the engine.

[chinmaygarde]

Ah, I guess I can also use --system-header-prefix. I don't think it is reasonable for stuff in third_party/ to have to adhere to our warning flags.

[chinmaygarde]

Fails on Fuchsia headers. Will retry again later.