Add comments explaining use of "-Wno-over-aligned"

Follow up to I04a5b013dd0ec8f162046388e89cf08cb9476bab

Change-Id: I5ab77564fe867ad1fef48fb322ff75869c025776
Reviewed-on: https://skia-review.googlesource.com/79040
Commit-Queue: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>

gn/gn_to_bp.py

@@ -76,6 +76,10 @@ cc_library_static {
                 $x86_srcs
             ],
             cflags: [
+                // Clang seems to think new/malloc will only be 4-byte aligned
+                // on x86 Android. We're pretty sure it's actually 8-byte
+                // alignment. tests/OverAlignedTest.cpp has more information,
+                // and should fail if we're wrong.
                 "-Wno-over-aligned"
             ],
         },