Make content/common compile on Linux GN build.

content/public/common/BUILD.gn

Index: content/public/common/BUILD.gn
diff --git a/content/public/common/BUILD.gn b/content/public/common/BUILD.gn
index 4788b6d15495f1162645acd19f234cbe91fc7d9c..562bf6d8104f3e8c572599719549bea95f24d42c 100644
--- a/content/public/common/BUILD.gn
+++ b/content/public/common/BUILD.gn
@@ -8,7 +8,16 @@ source_set("common") {
   sources = rebase_path(content_common_gypi_values.public_common_sources,
                         ".", "//content")
 
+  defines = [ "CONTENT_IMPLEMENTATION" ]
+
+  configs += [
+    # TODO(GYP) remove when blink_headers target is converted.
+    "//content:blink_headers_stub_config",
+  ]
+
   deps = [
     "//content/common",
+    "//skia",
+    #"//third_party/WebKit/public:blink_headers",
   ]
 }