Merge pull request #24233 from davidflanagan/bug1070195

Bug 1070195: free canvas memory after creating a video poster image in Camera r=wilsonpage(cherry picked from commit 3563ddca35a52e7a533dcfc7996e74b78c6e36c8)
Merge pull request #24240 from davidflanagan/bug1070195-v2.0

Merge pull request #24233 from davidflanagan/bug1070195