memcpy is faster than UTF-8 encoding/decoding. This yields 10-20% wins on
serializing and deserializing long ASCII strings, according to
blink_perf.bindings -- and these are already in a fast path where the entire
string is known to be ASCII (but this has to be checked). The win may be
larger for strings in Latin-1 but not ASCII (though I suspect this is an
uncommon case).

A change is also made to make ValueSerializerTest.EncodeTwoByteStringUsesPadding
survive wire format version number changes.

This is the first of a series of wire format changes from the previous Blink
format. The deserializer continues to be able to read the old format, but
Chromium M56 will no longer be able to read the messages written by this, in M58.

BUG=chromium:686159

Review-Url: https://codereview.chromium.org/2658793004
Cr-Commit-Position: refs/heads/master@{#42753}