fix: don't use deprecate-soon class v8::String::Value (#43553)

* fix: remove use of deprecated v8::String::Value

Upstream marked v8::String::Value as `V8_DEPRECATE_SOON` last month,
so let's stop using it.

The replacement code mostly does the same as v8::String::Value();
but since our test only cares about the length and not the contents,
we get a small perf win of not needing to allocate a char array and
not needing to call Local::String::Write().

Upstream V8_DEPRECATE_SOON:
Xref: https://chromium-review.googlesource.com/c/v8/v8/+/5667299kkk

v8::String::Value() implementation:
20226b740b/src/api/api.cc (10883)

History on why we used it:
80c1a9739d
f49ed30f72

Co-authored-by: Charles Kerr <charles@charleskerr.com>

* Update shell/common/gin_converters/file_path_converter.h

Co-authored-by: Robo <hop2deep@gmail.com>

Co-authored-by: Charles Kerr <charles@charleskerr.com>

* fixup! Update shell/common/gin_converters/file_path_converter.h

do not return success for all non-Null non-Strings

Co-authored-by: Charles Kerr <charles@charleskerr.com>

---------

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Charles Kerr <charles@charleskerr.com>

shell/common/gin_converters/file_path_converter.h

@@ -23,9 +23,8 @@ struct Converter<base::FilePath> {
     if (val->IsNull())
       return true;
 
-    v8::String::Value str(isolate, val);
-    if (str.length() == 0) {
-      *out = base::FilePath();
+    if (val->IsString() && v8::Local<v8::String>::Cast(val)->Length() == 0) {
+      *out = {};
       return true;
     }