Add osmesa to GN build.

content/shell/BUILD.gn

Index: content/shell/BUILD.gn
diff --git a/content/shell/BUILD.gn b/content/shell/BUILD.gn
index f6e7f4f3f16dc169f84ebdbc2e0d17a148a480d2..16d2c859b38682e67b6bd56254f402a2679d9e81 100644
--- a/content/shell/BUILD.gn
+++ b/content/shell/BUILD.gn
@@ -249,9 +249,9 @@ static_library("content_shell_lib") {
   }
 
   if (use_x11) {
-    # TODO(GYP) The GYP build includes a dep on the xdisplaycheck executable.
-    # Track down who added this and ask them if it's necessary, since this
-    # seems suspicious.
+    # Some tests rely on this tool at runtime. Note: it might be better if
+    # the tests that needed it had this as a dep instead of adding it here.
+    datadeps = [ "//tools/xdisplaycheck" ]
   }
 
   if (is_android) {
@@ -437,7 +437,7 @@ executable("content_shell") {
     ":content_shell_lib",
     ":pak",
     "//base/allocator",
-    #'../third_party/mesa/mesa.gyp:osmesa',  TODO(GYP)
+    "//third_party/mesa:osmesa",
   ]
 
   if (is_win) {
@@ -453,7 +453,7 @@ executable("content_shell") {
   }
 
   if (!is_android || !is_android_webview_build) {
-    deps += [
+    datadeps += [
       # NOTE: rely on host build of this target.
       #'../tools/imagediff/image_diff.gyp:image_diff',  TODO(GYP)
     ]