make error messages to be more consistent
ppb_flash_drm: remove redundant code
static_assert: add and use STATIC_ASSERT_LESS_OR_EQ()

to workaround code editor quirks
impl ppb_file_chooser_create
add ppb_var_add_ref2() which returns its argument after adding reference
impl ppb_file_ref_create_unrestricted
impl ppb_file_ref_get_name and ppb_file_ref_get_path
impl ppb_file_chooser_show_without_user_gesture
impl ppb_flash_file_file_ref_open_file
move ppb_flash_file_modulelocal.c into ppb_flash_file.c