Create file_manager_strings.grdp to manage independently

This patch creates file_manager_strings.grdp as collecting file
manager strings from chromeos_strings.grdp to manage
independently.

It also moves sha1 for file manager already added at
chrome/app/chromeos_strings_grdp to
chrome/app/file_manager_strings_grdp

It's wrapped in <if expr="chromeos"> at generated_resources.grd.

TEST=tested manually

Bug: 845818
Change-Id: I3741871b2fe9e0d0aec8da10caa7f4ab3f20363a
Reviewed-on: https://chromium-review.googlesource.com/1195173Reviewed-by: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#588255}

chrome/app/generated_resources.grd

@@ -188,6 +188,7 @@ are declared in tools/grit/grit_rule.gni.
       <!-- Chrome-OS-specific strings -->
       <if expr="chromeos">
         <part file="chromeos_strings.grdp" />
+        <part file="file_manager_strings.grdp" />
       </if>
 
       <!-- Settings specific strings -->