Bug 917226 - Build a canvas inspection tool, r=rcampbell

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extra : rebase_source : 466745d61633e1e56f7fd1f2e680952c52cdf0a4
Backed out changeset 9c456120ed57 (bug 917226) for browser-chrome failures

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extra : rebase_source : f2780e4762b1537a07100be52f6949b201f24dc1