Merge pull request #17837 from shamenchens/Bug967089-Remove-Visibility-Monitor-ImgSrc2

Bug 967089 - [gallery] remove visibility monitor code if it is no longer..., r=punam(cherry picked from commit 006cab98e7844de9adeb93f6faa8d33205c8b8f3)
Merge pull request #20457 from davidflanagan/bug1015413

Bug 1015413: resolve large thumbnail flicker by uplifting bug 967089 to remove visibility monitor