Added EncodedImage::GetBufferPaddingBytes.

The FFmpeg video decoder requires up to 8 additional bytes to be allocated for its encoded image buffer input, due to optimized byte readers over-reading on some platforms.
We plan to use FFmpeg for a soon-to-land H.264 enc/dec.

This CL adds support for padding encoded image buffers based on codec type, and makes sure calls to VCMEncodedFrame::VerifyAndAllocate use the padding.

All padding constants are 0 but making H.264 pad with 8 bytes will be a one-line change.

Also, added -framework CoreFoundation to webrtc_h264_video_toolbox which was missing.

BUG=chromium:468365
BUG=https://bugs.chromium.org/p/webrtc/issues/detail?id=5424
NOTRY=True

Review URL: https://codereview.webrtc.org/1602523004

Cr-Commit-Position: refs/heads/master@{#11337}

webrtc/modules/video_coding/codecs/h264/h264.gypi

@@ -50,6 +50,7 @@
           'link_settings': {
             'xcode_settings': {
               'OTHER_LDFLAGS': [
+                '-framework CoreFoundation',
                 '-framework CoreMedia',
                 '-framework CoreVideo',
                 '-framework VideoToolbox',