Make UTF16RagelIterator::operator*() to use DCHECK() instead of CHECK()

This patch changes |UTF16RagelIterator::operator*()| to use |DCHECK()| instead
of |CHECK()| for improving performance because this function is called many
times as number of characters in the page.

In text heavy page, I observe 12% speed up.

Change-Id: I5febb53f0f096b3bd76e6330a70b6464b9cb4758
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760846
Auto-Submit: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688476}

third_party/blink/renderer/platform/fonts/utf16_ragel_iterator.h

@@ -106,7 +106,7 @@ class PLATFORM_EXPORT UTF16RagelIterator {
   }
 
   UChar32 operator*() {
-    CHECK(buffer_size_);
+    DCHECK(buffer_size_);
     return cached_category_;
   }