[Regression fix] [Data URI parser] Accept data URI with invalid mediatype data

[Regression fix] [Data URI parser] Accept data URI with invalid mediatype data

r272022 introduced grammar check on the mediatype part in a data URI.
This broke applications that depend on the behavior that our
WebURLLoaderImpl and URLRequestDataJob accept data URIs with
incomplete mediatype such as "data:image;base64,xxx". Accept this kind
of data URI again for backward compatibility.

Instead, fallback to the default mediatype when
ParseMimeTypeWithoutParameter() returns false in DataURL::Parse() to
avoid generating Content-type header with invalid mediatype.

BUG=412479
Review URL: https://codereview.chromium.org/555383003

Cr-Commit-Position: refs/heads/master@{#297593}
(cherry picked from commit 5469386eaa6bd55fcd5540c570fb535f4d6667e3)

Committed: https://chromium.googlesource.com/chromium/src/+/20b5436e589ee79b30a62576dae00ead4cc41136

[tyoshino (SeeGerritForStatus), 2014-10-23 08:03:07]

Committed patchset #1 (id:1) manually as
20b5436e589ee79b30a62576dae00ead4cc41136.