media/base VideoFrameLayout: Update comment about stride

The original comment about stride in VideoFrameLayout::Plane is confusing as its
value is in bytes or in elements. This clarifies that it stands for a stride in
bytes.

Bug: None
Test: Confirm stride is used as bytes everywhere for now
Change-Id: I9eae65c77add74ad07bba936bc315f33ea4c7bc5
Reviewed-on: https://chromium-review.googlesource.com/c/1477627Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634507}

media/base/video_frame_layout.h

@@ -45,9 +45,8 @@ class MEDIA_EXPORT VideoFrameLayout {
     bool operator==(const Plane& rhs) const;
     bool operator!=(const Plane& rhs) const;
 
-    // Strides of a plane, typically greater or equal to the
-    // width of the surface divided by the horizontal sampling period. Note that
-    // strides can be negative if the image layout is bottom-up.
+    // Strides in bytes of a plane. Note that stride can be negative if the
+    // image layout is bottom-up.
     int32_t stride = 0;
 
     // Offset of a plane, which stands for the offset of a start point of a