media: Require SecureContext for EME APIs
In this CL requestMediaKeySystemAccess() requires SecureContext. On
non-SecureContext, this API will not be visible.
Some notes for future reference:
1. file:// is always treated as SecureContext.
2. file:// is not a unique origin. The origin string is "file://"
3. LoadFromData is treated as non-SecureContext. See the updated 
in this CL.
4. LoadFromData is also treated as unique origin. This is what 
used to test without this CL.
5. allowFileAccessFromFileURLs has nothing to do with SecureContext.
Rather it controls whether you can load other files from file://.
- So if you run a simple html page on file://, and that file
doesn't depend on any other file, you don't need
allowFileAccessFromFileURLs. This is why for  we don't
really need allowFileAccessFromFileURLs.
- If the html page depends on other js files, without
allowFileAccessFromFileURLs, those js files will not be loaded.
This will cause a lot of test failures. That's why in content
shell, we have allowFileAccessFromFileURLs enabled by default.
TEST=Manually tested and made sure EME APIs are not available on
12 files changed