Blob/File constructors/slice method shouldn't throw on invalid types

Blob/File constructors/slice method shouldn't throw on invalid types

Per spec[1] and tests[2] the constructors for Blob and File and their
slice() methods should not throw if non-ASCII type is supplied;
instead it should use "" as the type. In addition, the type should be
consistently lower-cased, by both the constructors and slice() method.

[1] https://w3c.github.io/FileAPI/
[2] https://github.com/w3c/web-platform-tests/tree/master/FileAPI

BUG=509786
R=kinuko@chromium.org

[jsbell, 2015-09-23 17:34:11]

migrated from https://codereview.chromium.org/1362963003/