Improving readability of image_data.cc

There is lots of magic number 4 in the image_data.cc. in fact, we are
using 4 to represent that there are 4 color bit to describe each pixel,
namely, red, green, blue and alpha. Add a class level comment to
describe what 4 means in image_data.cc.

Bug: 1122866

Change-Id: I728ea5fadba3bc7f7477dbbdd35aa180872beb84
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379656Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Yi Xu <yiyix@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803954}

third_party/blink/renderer/core/html/canvas/image_data.cc

@@ -39,6 +39,8 @@
 
 namespace blink {
 
+// Please note that all the number "4" in the file means number of channels
+// required to describe a pixel, namely, red, green, blue and alpha.
 namespace {
 
 bool RaiseDOMExceptionAndReturnFalse(ExceptionState* exception_state,