Mechanical and structural fixes to avoid unnecessary allocations,
copies, and V8 boundary crossings in gin converter and gin_helper hot
paths.

gin_converters/std_converter.h:
- std::set/std::map FromV8: move the converted element/key into the
  container instead of copying.
- std::set/std::span ToV8: use CreateDataProperty() (the direct define
  path) instead of Object::Set() on freshly-created arrays. The
  NOTREACHED message already referenced CreateDataProperty.
- std::map FromV8: hoist keys->Length() out of the loop.

gin_converters/net_converter.{h,cc}:
- HttpResponseHeaders*::ToV8: build the V8 {name: [value, ...]} object
  directly from a std::map<string, vector<string>> instead of
  round-tripping through base::DictValue (a flat_map with O(d) shift on
  new-key insert) plus content::V8ValueConverter (which allocates and
  walks an entirely separate base::Value graph).
- ResourceRequestBody::ToV8: hoist GetCurrentContext() out of the loop;
  use CreateDataProperty() on the freshly-created array.
- vector<pair<K,V>>::FromV8: hoist keys->Length(); reserve().
- VerifyRequestParams::ToV8: take by const& instead of by value.

gin_converters/blink_converter.cc:
- WebKeyboardEvent::ToV8: cache GetModifiers() once instead of calling
  it nine times.
- ModifiersToArray: reserve() the result vector.

gin_converters/gfx_converter.cc:
- ColorSpace::ToV8: use std::string_view for the four enum-name
  variables that are always assigned compile-time literals.

gin_converters/guid_converter.h:
- Uuid::FromV8: ParseCaseInsensitive() instead of
  ParseLowercase(ToLowerASCII()) — avoids a temporary string.
- Uuid::ToV8: take by const& and pass AsLowercaseString() through
  directly — avoids two heap-allocated string copies.

gin_converters/login_item_settings_converter.{h,cc}:
- LaunchItem/LoginItemSettings ToV8: take by const& (each holds strings
  and vectors).

gin_helper/dictionary.h:
- SetHidden / SetReadOnlyNonConfigurable: take T by const& to match the
  sibling Set/SetReadOnly signatures.
- SetGetter accessor lambda: pass acc_value.Value directly to
  TryConvertToV8 instead of copying it to a local first.

Microbenchmark medians (Linux x86-64 testing build):

  HttpResponseHeaders::ToV8 (16 lines)  30790 ns -> 19020 ns  (-38%)
  Uuid::ToV8                              320 ns ->    92 ns  (-71%)
  Uuid::FromV8 (mixed-case)               561 ns ->   425 ns  (-24%)
  Dictionary::SetHidden+SetReadOnlyNonCfg 3147 ns ->  2627 ns (-17%)
  span<int>::ToV8 (64 ints)             12027 ns -> 10311 ns  (-14%)
  set<string>::ToV8 (11 strings)         3857 ns ->  3485 ns  (-10%)
  WebKeyboardEvent::ToV8                15883 ns -> 15428 ns  (-3%)
  vector<pair>::FromV8 (10 keys)         6288 ns ->  6241 ns  (-1%)
  set<string>::FromV8                    3549 ns ->  3563 ns  (no change)
  map<string,string>::FromV8             7833 ns ->  7820 ns  (no change)
  gfx::ColorSpace::ToV8                  4842 ns ->  4845 ns  (no change)

Co-authored-by: Sam Attard <sattard@anthropic.com>