base: Remove const modifier from return type of DiscardableMemory::data_as().
base: Remove const modifier from return type of DiscardableMemory::data_as().

To allow this helper function to also be used for writing to
discardable memory region.

BUG=

Review URL: https://codereview.chromium.org/1100463002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#326362}

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