Fix JNI reference leak in MediaCodecVideoDecoder
Fix JNI reference leak in MediaCodecVideoDecoder

We currently leak one local reference to MediaCodecVideoDecoder in
every call to MediaCodecVideoDecoderFactory::CreateVideoDecoder. After
the decoder has been re-initialized 512 times, JNI will crash due to
local reference table overflow (max=512).

The actual leak is in the member initializer list of
MediaCodecVideoDecoder. This CL fixes the leak by adding a
ScopedLocalRefFrame outside of the ctor. All JNI code that originate
from a C++ thread (i.e. the entry point is not a Java thread) must use
a ScopedLocalRefFrame in order to avoid leaking local references.

BUG=webrtc:6969,b/36713034

Review-Url: https://codereview.webrtc.org/2780273002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#17464}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e4cd15d379b1a0bf1f91aaf873cfa07b0998f295

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