Reallocate encoded buffer size if needed for VP8. Initially set to the input image size.

Issue may occur for very small input images (e.g. 4x4) when encoded image length > input image size.

BUG=chromium:571594

Review URL: https://codereview.webrtc.org/1626373002

Cr-Commit-Position: refs/heads/master@{#11376}

webrtc/modules/video_coding/codecs/vp8/simulcast_unittest.h

@@ -610,6 +610,8 @@ class TestVp8Simulcast : public ::testing::Test {
 
   void TestSwitchingToOneOddStream() { SwitchingToOneStream(1023, 769); }
 
+  void TestSwitchingToOneSmallStream() { SwitchingToOneStream(4, 4); }
+
   void TestRPSIEncoder() {
     Vp8TestEncodedImageCallback encoder_callback;
     encoder_->RegisterEncodeCompleteCallback(&encoder_callback);