Backport - easy fixes for memory leaks investigation

Bug 1234343 - Overly large values reported for gfx/heap-textures (changes size_t to ptrdiff_t (which "was fixed" from int32_t by previous bug) in FF46 (and Mozilla has closed related ticket with that confirmed bug as WORKSFORME for FF45, ugh Bug 1262088 - Incorrect about:memory reports (gfx/heap-textures) when playing HTML5 videos) Without it TBB shows

│  ├──3,155.00 MB (93.38%) ── heap-textures

(As heap-textures causes a half of memory leaks, the helper ticket is worth tracking Bug 1290100 - Decompose explicit/gfx/heap-textures)

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