1.
DCHECK in runtime-i18n.cc for case mapping was wrong to
assume that the longest primary language tag is 3 characters.
BCP 47 actually allows up to 8 characters.

2. GetFlatContent() was called to a string without flattening it first.

BUG=680314,680464
TEST=intl/general/case-mapping (see also the bugs)

Review-Url: https://codereview.chromium.org/2629763003
Cr-Commit-Position: refs/heads/master@{#42343}