Make content_gpu.gypi depend on content_common_mojo_bindings in gyp.

mirroring https://code.google.com/p/chromium/codesearch#chromium/src/content/gpu/BUILD.gn&l=53

BUG=581707
NOTRY=true
NOTREECHECKS=true
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/1648503002

Cr-Commit-Position: refs/heads/master@{#371910}

content/content_gpu.gypi

@@ -7,6 +7,7 @@
     '../base/base.gyp:base',
     '../skia/skia.gyp:skia',
     '../ui/gl/gl.gyp:gl',
+    'content_common_mojo_bindings.gyp:content_common_mojo_bindings',
   ],
   'sources': [
     'gpu/gpu_child_thread.cc',