DevTools: introduce uiSourceCode.mimeType() method

This patch introduces the uiSourceCode.mimeType() method instead
of static NetworkProject.uiSourceCodeMimeType().

This is necessary to separate uiSourcecode's mimetypes from
NetworkProject. This is also natural since UISourceCode's mimeType
is known by its creator.

R=dgozman
BUG=670180

Review-Url: https://codereview.chromium.org/2889013002
Cr-Commit-Position: refs/heads/master@{#473325}
20 files changed