[Backport] content_child: Explicitly depend on blink_image_resources.

Commit a3c614c ("Part 4 of ??: Move blink images to their own grd/pak
files") introduced a dependency on the newly-generated blink_resources.h
header in blink_platform_impl.cc, but did not explicitly made the
content_child target depend on the target that generated it.

Other targets elsewhere work fine because they depend on one or more pak
files also generated by the grit invocation, but that is not the case
for content_child, which only uses the header that's created.

BUG=467176, 471609
TEST=ninja -C out/Release content_child [from scratch]
R=erg@chromium.org, jam@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/1088513006

Author: raphael.kubo.da.costa

content/content_child.gypi

@@ -386,6 +386,7 @@
         'app/strings/content_strings.gyp:content_strings',
         '../storage/storage_common.gyp:storage_common',
         '../third_party/WebKit/public/blink.gyp:blink',
+        '../third_party/WebKit/public/blink_resources.gyp:blink_image_resources',
         '../third_party/WebKit/public/blink_resources.gyp:blink_resources',
         '../third_party/npapi/npapi.gyp:npapi',
       ],