Enable @CheckDiscard for fields.

The annotation doc already mentions they are supported.

Verified locally that the check is triggered when an annotated field is
optimized out.

Change-Id: Ida56c91238afc2141c5562a49544a27afe9af452
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918332
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715573}

base/android/java/src/org/chromium/base/annotations/CheckDiscard.java

@@ -15,7 +15,7 @@ import java.lang.annotation.Target;
  *
  * Note that @RemovableInRelease implies @CheckDiscard.
  */
-@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.TYPE})
+@Target({ElementType.CONSTRUCTOR, ElementType.FIELD, ElementType.METHOD, ElementType.TYPE})
 @Retention(RetentionPolicy.CLASS)
 public @interface CheckDiscard {
     /**