Reserve Vector's capacity manually in CachedMetadata

As CachedMetaData::m_serializedData is set only at the initialization timing,
we can call Vector<char>::reserveCapacity to save memory consumption without
loosing performance.

This CL includes other cleanups:

 - Renaming methods,
 - Replacing error handling logic with DCHECKs,
 - Replacing unsigned with uint32_t.

BUG=636462

Review-Url: https://codereview.chromium.org/2258743002
Cr-Commit-Position: refs/heads/master@{#415544}
9 files changed