[Files.app] Add a shortcut key (Ctrl-Shift-E) to eject a removable device

BUG=459693
TEST=manually tested

Review URL: https://codereview.chromium.org/954113004

Cr-Commit-Position: refs/heads/master@{#318179}

ui/file_manager/file_manager/main.html

@@ -218,7 +218,8 @@
       <command id="volume-switch-8" shortcut="U+0038-Ctrl">
       <command id="volume-switch-9" shortcut="U+0039-Ctrl">
 
-      <command id="unmount" i18n-values="label:UNMOUNT_DEVICE_BUTTON_LABEL">
+      <command id="unmount" i18n-values="label:UNMOUNT_DEVICE_BUTTON_LABEL"
+               shortcut="U+0045-Shift-Ctrl"><!-- Shortcut: 'Shift-Ctrl-E' -->
       <command id="format" i18n-values="label:FORMAT_DEVICE_BUTTON_LABEL">
 
       <command id="volume-help" i18n-values="label:DRIVE_MENU_HELP">