Pass lambda to endPage callback which read back textures

Expecting presubmit to pass after landing
https://skia-review.googlesource.com/c/skia/+/344158

Bug:skia:9765
Test: ran capture on a Pixel 3 and examined file in skia debugger

Change-Id: I3153c79fe5b98cd8cfdff928ae4940dc4a4f0b88