Nil out EAGLContext explicitly on RTCEAGLVideoView dealloc.
Nil out EAGLContext explicitly on RTCEAGLVideoView dealloc.

Theoretical fix to address some EAGLContext issues from other UIImageViews that could be active.

NOTRY=True
BUG=

Review-Url: https://codereview.webrtc.org/2259513002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13806}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 41a32874727160d3309ba5b1783a4c87a7fd9d64

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