[wasm] Set flutter_runtime_mode in tools/gn to_gn_wasm_args #193406

Summary

The engine/src/flutter/tools/gn build script ignores the --runtime-mode argument for WebAssembly (--web / wasm) targets. As a result, Wasm builds (including wasm_release / skwasm / Web Impeller) implicitly evaluate flutter_runtime_mode as "debug" globally inside GN even when --runtime-mode=release is passed.

For Web Impeller (Wimp), this evaluates impeller_debug = true (IMPELLER_DEBUG=1), causing debug tracing mechanisms (such as RenderPassGLES::EncodeCommandsInReactor executing synchronous gl.CheckFramebufferStatusDebug(GL_FRAMEBUFFER) 50+ times per frame) to pollute release artifacts and cause severe frame-bound GPU stalls in release Wimp targets.

Steps to Reproduce

1. Execute engine/src/flutter/tools/gn --web --runtime-mode=release
2. Search out/wasm_release/args.gn or inspect the GN evaluation tree.

Observed Behavior

flutter_runtime_mode defaults to "debug", activating IMPELLER_DEBUG=1.

Expected Behavior

flutter_runtime_mode evaluates to "release", de-activating IMPELLER_DEBUG and yielding an optimized Wasm payload free of synchronous GL verification commands.

Environment / Target

• Target OS: wasm (--web / wimp / skwasm)
• Build Script: engine/src/flutter/tools/gn (to_gn_wasm_args)