media/gpu/linux: fix typo in frame mapper description

Fix a typo in CreateMappedVideoFrame()'s documentation: no memory is
deallocated by the destruction observer, we are simply removing a
mapping to the user process address space.

Bug: 856562
Test: None

Change-Id: I7be56440a4a743a3f78cc9a0c60aec27881c2e57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610663
Auto-Submit: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659389}

media/gpu/linux/generic_dmabuf_video_frame_mapper.cc

@@ -36,7 +36,7 @@ void MunmapBuffers(const std::vector<std::pair<uint8_t*, size_t>>& chunks,
   }
 }
 
-// Create VideoFrame whose dtor deallocates memory in mapped planes referred
+// Create VideoFrame whose dtor unmaps memory in mapped planes referred
 // by |plane_addrs|. |plane_addrs| are addresses to (Y, U, V) in this order.
 // |chunks| is the vector of pair of (address, size) to be called in munmap().
 // |src_video_frame| is the video frame that owns dmabufs to the mapped planes.