media: make camera metadata tags mojo interface extensible

Make camera metadata tags mojo interface so that it matches the HAL
adapter mojo modified for portrait mode (Bokeh).

BUG=b:79901482
TEST=CCA works on Soraka
R=jcliang@chromium.com

Change-Id: I5aaf263efa863f569477cf30a4a63b93f242ab30
Reviewed-on: https://chromium-review.googlesource.com/1195187Reviewed-by: Ricky Liang <jcliang@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Hung-yu Wu <hywu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587875}

media/capture/video/chromeos/mojo/camera_metadata_tags.mojom

@@ -112,6 +112,7 @@ enum CameraMetadataSectionStart {
  * addition, the name and type of the tag needs to be added to
  * system/media/camera/src/camera_metadata_tag_info.c
  */
+[Extensible]
 enum CameraMetadataTag {
     ANDROID_COLOR_CORRECTION_MODE =                    0x0, // ANDROID_COLOR_CORRECTION_START,
     ANDROID_COLOR_CORRECTION_TRANSFORM,