Fix ENABLE_SWIFTSHADER flag.

Convert it to a build flag, as it never properly made the move from gyp.

BUG=646660

Review-Url: https://codereview.chromium.org/2337243003
Cr-Commit-Position: refs/heads/master@{#418948}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index e4f9f277..af7034c 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -1443,6 +1443,7 @@
     "//ui/gfx",
     "//ui/gfx/geometry",
     "//ui/gl",
+    "//ui/gl:gl_features",
     "//ui/message_center",
     "//ui/resources",
     "//ui/resources",