[clang-tidy] Use explicit default

See
https://chromium.googlesource.com/chromium/src/+/HEAD/styleguide/c++/c++-dos-and-donts.md#prefer-to-use.

Bug: v8:10488
Change-Id: I4416c2bfec08474b805395b95b58a877ddbdb2fe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2228410Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68183}

src/d8/d8.cc

@@ -433,7 +433,7 @@ static platform::tracing::TraceConfig* CreateTraceConfigFromJSON(
 class ExternalOwningOneByteStringResource
     : public String::ExternalOneByteStringResource {
  public:
-  ExternalOwningOneByteStringResource() {}
+  ExternalOwningOneByteStringResource() = default;
   ExternalOwningOneByteStringResource(
       std::unique_ptr<base::OS::MemoryMappedFile> file)
       : file_(std::move(file)) {}