Disable update of last_show_frame for existing frame

When showing a previously decoded frame, i.e. when
show_existing_frame=1, the update of the
last_show_frame flag must be disabled.

This is to ensure that the last_show_frame flag
reflects the state of the flag for the immediately
previously decoded frame rather then the value that
was forced to ensure that a previously decoded frame
would be displayed.

This patch also adds a test vector to verify that the
display_existing_frame flag works as expected. Code
for generating the test vector can be found in this
patch:
https://gerrit.chromium.org/gerrit/#/c/68581/

(Bug originally reported by Alexander Voronov
<ru.xalba@gmail.com>).

Change-Id: I731d288fba02088959f7fcc87707137fffc6acf5

test/test-data.sha1

diff --git a/test/test-data.sha1 b/test/test-data.sha1
index 6daf69e..a287731 100644
--- a/test/test-data.sha1
+++ b/test/test-data.sha1
@@ -574,3 +574,5 @@
 84c1599298aac78f2fc05ae2274575d10569dfa0  vp90-2-09-aq2.webm.md5
 55fc55ed73d578ed60fad05692579873f8bad758  vp90-2-09-lf_deltas.webm
 54638c38009198c38c8f3b25c182b709b6c1fd2e  vp90-2-09-lf_deltas.webm.md5
+510d95f3beb3b51c572611fdaeeece12277dac30  vp90-2-10-show-existing-frame.webm
+14d631096f4bfa2d71f7f739aec1448fb3c33bad  vp90-2-10-show-existing-frame.webm.md5

test/test.mk

diff --git a/test/test.mk b/test/test.mk
index cb62615..a65decf 100644
--- a/test/test.mk
+++ b/test/test.mk
@@ -676,6 +676,8 @@
 LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += vp90-2-09-lf_deltas.webm.md5
 LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += vp91-2-04-yv444.webm
 LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += vp91-2-04-yv444.webm.md5
+LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += vp90-2-10-show-existing-frame.webm
+LIBVPX_TEST_DATA-$(CONFIG_VP9_DECODER) += vp90-2-10-show-existing-frame.webm.md5
 
 ifeq ($(CONFIG_DECODE_PERF_TESTS),yes)
 # BBB VP9 streams

test/test_vectors.cc

diff --git a/test/test_vectors.cc b/test/test_vectors.cc
index 16298d0..3227f52 100644
--- a/test/test_vectors.cc
+++ b/test/test_vectors.cc
@@ -160,6 +160,7 @@
   "vp90-2-02-size-lf-1920x1080.webm",
   "vp90-2-09-aq2.webm",
   "vp90-2-09-lf_deltas.webm",
+  "vp90-2-10-show-existing-frame.webm",
 #if CONFIG_NON420
   "vp91-2-04-yv444.webm"
 #endif

test/test_vectors.h

diff --git a/test/test_vectors.h b/test/test_vectors.h
index 5f62e99..eb592de 100644
--- a/test/test_vectors.h
+++ b/test/test_vectors.h
@@ -22,9 +22,9 @@
 
 #if CONFIG_VP9_DECODER
 #if CONFIG_NON420
-const int kNumVp9TestVectors = 216;
+const int kNumVp9TestVectors = 217;
 #else
-const int kNumVp9TestVectors = 215;
+const int kNumVp9TestVectors = 216;
 #endif
 
 extern const char *kVP9TestVectors[kNumVp9TestVectors];