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This make the API more consistent with RefCountedMemory and
other discardable memory interfaces such as SkDiscardableMemory.

Also improves readability by replacing memory->Memory() calls
with memory->data() calls.

A handy method is also added to simplify calling data() with a
reinterpret_cast. This is also consistent with RefCountedMemory.

BUG=

Review URL: https://codereview.chromium.org/1094433002

Cr-Commit-Position: refs/heads/master@{#325509}