Lock generator_mutex_ inside DecodeFailed

Bug: 962126

We typically lock this mutex before reading or writing to
decode_failed_. Do so in one more place to avoid a data race.

Change-Id: I0248e4dd99defc5ad3d09575096847e6455adfc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610085Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Auto-Submit: Leon Scroggins <scroggo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659475}

third_party/blink/renderer/platform/graphics/image_frame_generator.h

@@ -103,7 +103,10 @@ class PLATFORM_EXPORT ImageFrameGenerator final
   SkISize GetSupportedDecodeSize(const SkISize& requested_size) const;
 
   bool IsMultiFrame() const { return is_multi_frame_; }
-  bool DecodeFailed() const { return decode_failed_; }
+  bool DecodeFailed() const {
+    MutexLocker lock(generator_mutex_);
+    return decode_failed_;
+  }
 
   bool HasAlpha(size_t index);
 
@@ -149,7 +152,7 @@ class PLATFORM_EXPORT ImageFrameGenerator final
   const std::vector<SkISize> supported_sizes_;
 
   // Prevents concurrent access to all variables below.
-  Mutex generator_mutex_;
+  mutable Mutex generator_mutex_;
 
   bool decode_failed_ = false;
   bool yuv_decoding_failed_ = false;