mac/gn: Unbreak video_encode_accelerator_unittest after https://codereview.chromium.org/1818903004

A media/gpu header now includes a webrtc video_common header, so video_common
now must be a public_dep, so that webrtc's include directories are passed to
dependencies.

BUG=616220
NOTRY=true

Review-Url: https://codereview.chromium.org/2028543003
Cr-Commit-Position: refs/heads/master@{#397114}

media/gpu/BUILD.gn

@@ -186,9 +186,9 @@ component("gpu") {
                ] + get_target_outputs(":libvt_generate_stubs")
     deps += [
       ":libvt_generate_stubs",
-      "//third_party/webrtc/common_video",
       "//third_party/webrtc/system_wrappers",
     ]
+    public_deps += [ "//third_party/webrtc/common_video" ]
     libs += [
       "CoreFoundation.framework",
       "Foundation.framework",