Allocation must respect requested reserve size, not requested commit area size.

Additionally fix check failures in debug mode for mips64.
After CL: https://codereview.chromium.org/1147503002/
memory for large code objects is allocated from code range,
memory for regular code objects is allocated normally.

TEST=cctest/test-spaces/MemoryChunk
BUG=

Review URL: https://codereview.chromium.org/1172333002

Cr-Commit-Position: refs/heads/master@{#28995}