Remove dead file MockDiscardablePixelRef.h from gypi file.
Remove dead file MockDiscardablePixelRef.h from gypi file.

Source/platform/graphics/test/MockDiscardablePixelRef.h was removed in
https://codereview.chromium.org/485833005 and this removes the file
from blink_platform.gypi as well to avoid warnings in the msvs-ninja
gyp generator.

BUG=459058
R=senorblanco@chromium.org,hclam@chromium.org

Review URL: https://codereview.chromium.org/952443003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190671 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Bysmyyr/blink-crosswalk,Pluto-tv/blink-crosswalk,kurli/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,smishenk/blink-crosswalk,kurli/blink-crosswalk,smishenk/blink-crosswalk,PeterWangIntel/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,kurli/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,kurli/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,PeterWangIntel/blink-crosswalk,Bysmyyr/blink-crosswalk,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk,kurli/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,Bysmyyr/blink-crosswalk,kurli/blink-crosswalk,smishenk/blink-crosswalk,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,kurli/blink-crosswalk,kurli/blink-crosswalk,Pluto-tv/blink-crosswalk,smishenk/blink-crosswalk,kurli/blink-crosswalk,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,smishenk/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk,kurli/blink-crosswalk