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Adds the REQUIRE_MOCKS flag to GEN_JNI. If both the REQUIRE_MOCKS and
TESTING_ENABLED flags are set, calls to native implementations must
have had a mock instance set first through TEST_HOOKS,
otherwise an UnsupportedOperationException will be thrown.

If only TESTING_ENABLED is set, calls to the native implementation
will use a test mock if it is provided, and otherwise fallback to the
native implementation. This is used for instrumentation tests that
may want to mock out some native implementations but not all.

In the annotation processor version of GEN_JNI both these flags are
mutable and so they can be set by a JNI test rule for unit tests.

Instrumentation tests that want to mock a native implementation
can pass flags to JNI registration generator to set the corresponding
GEN_JNI flags.

Bug: 898261
Change-Id: I6a90e91a29368df692d458f35c9ea92f8898064b
Reviewed-on: https://chromium-review.googlesource.com/c/1344554
Commit-Queue: Aiden Benner <abenner@google.com>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610263}