Add bounds CHECK to UTF-8 decoder memory allocation.

Avoid integer overflow when computing a total buffer size from a base
buffer and small partial sequence buffer.

Bug: 901030
Change-Id: Ic82db2c6af770bd748fb1ec881999d0dfaac30f0
Reviewed-on: https://chromium-review.googlesource.com/c/1313833
Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605011}
std::unique_ptr<TextCodec> TextCodecUTF8::Create(const TextEncoding&,
                                                 const void*) {
  return base::WrapUnique(new TextCodecUTF8());
}

std::unique_ptr<TextCodec> TextCodecUTF8::Create(const TextEncoding&,
                                                 const void*) {
  return base::WrapUnique(new TextCodecUTF8());
}
