Fix comment typo in required_previous_frame+index_

Change '+' to '_' in that comment. It should read
required_previous_frame_index_.

Change-Id: I98e3a180be2d4fc3eaba6b01f9f80ec363da088b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153696Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
Cr-Commit-Position: refs/heads/master@{#760108}

third_party/blink/renderer/platform/image-decoders/image_decoder.h

@@ -412,7 +412,7 @@ class PLATFORM_EXPORT ImageDecoder {
   //
   // Image formats which do not use more than one frame do not need to
   // worry about this; see comments on
-  // ImageFrame::required_previous_frame+index_.
+  // ImageFrame::required_previous_frame_index_.
   size_t FindRequiredPreviousFrame(size_t frame_index,
                                    bool frame_rect_is_opaque);