Bug 1098185 - Initialize some more fields in ShadowLayerForwarder when Valgrind is enabled. r=mattwoodrow.

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extra : rebase_source : 0063948d7a657421005535c8faad68e69b1a5fcf
Bug 1098618 - Use a smaller initial IdSet size in jsiter.cpp:Snapshot(). r=sfink.

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extra : rebase_source : b506099d2a57f0c0199e2bd2713309630ebe03d3