Add FileSystemDirectoryHandle support

[itstorque]

Instead of uploading the entire Symbol directory, find a way to use FileSystemDirectoryHandle to have session based access. The problem is lib/sym is stored in Library which is excluded in here:

#if defined(OS_MAC)
    // Similar Mac specific blocks.
    ...
    {base::DIR_HOME, FILE_PATH_LITERAL("Library"), kBlockAllChildren},
#endif

[itstorque]

This can be added for Linux and Windows (see canvas.js in 565fa3f for separation based on OS). Options for Mac support would be:

• Make user move directory manually
• Make a user side script that updates
• Don't allow/recommend this by default, users opt-in to move directory

In 565fa3f, this is handled by not recommending it for Mac users. However, the current version also has a prebuilt standard library of components. In general this might be easier?