Fix discardable memory bot by calling lockPixels()

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-SK_USE_DISCARDABLE_SCALEDIMAGECACHE

BUG=skia:

Change-Id: I684226544f0e1f0234257ba88f6f2e56c32d5706
Reviewed-on: https://skia-review.googlesource.com/9783
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
1 file changed