Budget fStorage in SkPngCodecBase::allocateStorage

Replace `skia_private::AutoTMalloc<uint8_t> fStorage` in
`SkPngCodecBase` with `SkCodecPriv::BudgetedBuffer fStorage` so that
the color-transform scratch buffer allocated in
`SkPngCodecBase::allocateStorage()` is charged against
`fMaxDecodeMemory` via `allocateFromBudget()` and uses non-throwing
`sk_malloc_canfail`.

Bug: b/565484678
Change-Id: I1c0e83bcb84f69cae344b25a4b8e40d655fd321c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1375556
Commit-Queue: Elliot Sisteron <elliotsisteron@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>