This will be used as the foundation for writing a list of
MinidumpUTF8String objects. MinidumpUTF8String (and UTF-16
MINIDUMP_STRING) objects are never referenced by
MINIDUMP_LOCATION_DESCRIPTOR because they carry their own lengths.
Instead, they are always referenced by RVA.
The list of MinidumpUTF8String objects will be used for the module
annotations vector.
TEST=minidump_test MinidumpRVAListWriter.*
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/704333002
