Bug 977921 - Override ASan malloc_context_size to save memory. r=decoder

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extra : rebase_source : 500882c214de079c1d493e21a9ee91c3da743226
Bug 939562 part 3 - Move JIT flags from ContextOptions to RuntimeOptions. r=bent,bholley,luke

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extra : rebase_source : e25c6f5f381a972c472fb8a845c737fb31617682