Allow constructing an EglBase from an existing shared EGLContext.
Allow constructing an EglBase from an existing shared EGLContext.

BUG=None

Review-Url: https://codereview.webrtc.org/2885163003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#18266}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc