Automated g4 rollback of changelist 60108979.

*** Reason for rollback ***

Compile testing is open source and needs to run under jdk6.

*** Original change description ***

Use the Charset constants in the JDK's StandardCharsets class instead of c.g.c.base.Charsets.
The c.g.common.base.Charsets is scheduled for deletion.
More information: []ATTENTION REVIEWERS: This CL will cause no behavior changes.

Tested:
TAP presubmits for global presubmit queue
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=60789697