Make it clear that download.filename is ignored if OnDeterminingFilename

listener is registered.

If an extension registers an OnDeterminingFilename listener,
download.filename will be ignored as the listeners will provide
the filename.

BUG=1106297

Change-Id: Id44f5c7043f0e899f33f53f80458a00b162e5c99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318445
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792769}

chrome/common/extensions/api/downloads.idl

@@ -28,7 +28,9 @@ namespace downloads {
     // The $(ref:DownloadItem)'s new target $(ref:DownloadItem.filename), as a path
     // relative to the user's default Downloads directory, possibly containing
     // subdirectories. Absolute paths, empty paths, and paths containing
-    // back-references ".." will be ignored.
+    // back-references ".." will be ignored. <code>filename</code> is ignored if
+    // there are any $(ref:onDeterminingFilename) listeners registered by any
+    // extensions.
     DOMString filename;
 
     // The action to take if <code>filename</code> already exists.