Always check against SegmentStream cleared state

Previous, the calls to check IsCleared() were inside a DCHECK(). We want
to promote them to always happen.

This patch does that.

BUG=897263

Change-Id: I385b4bfeb931b94e62cd88258da8873f9ddff8f9
Reviewed-on: https://chromium-review.googlesource.com/c/1315703Reviewed-by: Leon Scroggins <scroggo@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605050}

third_party/blink/renderer/platform/image-decoders/segment_stream.cc

@@ -31,7 +31,8 @@ bool SegmentStream::IsCleared() const {
 }
 
 size_t SegmentStream::read(void* buffer, size_t size) {
-  DCHECK(!IsCleared());
+  if (IsCleared())
+    return 0;
 
   size = std::min(size, reader_->size() - position_);
 
@@ -48,7 +49,8 @@ size_t SegmentStream::read(void* buffer, size_t size) {
 }
 
 size_t SegmentStream::peek(void* buffer, size_t size) const {
-  DCHECK(!IsCleared());
+  if (IsCleared())
+    return 0;
 
   size = std::min(size, reader_->size() - position_);