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src: Fix inefficient usage of v8_inspector::StringView

Rodrigo Muino Tomonari requested to merge github/fork/szuend/fix-inspector-string-view-usage into main

v8_inspector::StringView can either be one-byte or two-byte strings. Node has currently two places where it's unconditionally assumed that it's a two-byte StringView.

This requires the upstream V8 inspector to unnecessarily create a copy of the string:

https://source.chromium.org/chromium/chromium/src/+/main:v8/src/inspector/v8-inspector-session-impl.cc;l=192-199;drc=05bacd38e7a31e92afe0fd66081dfa2cc03b934a

This is particularly slow, especially for large CDP messages, as the serialized JSON is iterated 8-bit char by 8-bit char and each one widened to a 16-bit char.

This PR introduces a small helper that correctly converts a StringView to a v8::String instance honoring the "is8Bit" flag. This allows upstream V8 to remove the unnecessary widening copy.

Tested locally with removing the widening copy in v8-inspector-session-impl.cc: Without the PR lots of tests will fail. But I'm happy to add unit tests for the JSBindingsConnection/V8ProfilerConnection if desired.

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