[actionbar] Change share menu cr-button to button

The share menu button was using 'cr-button', while the other menu
hosting buttons on the action bar were changed to use 'button'.
See CL:2143415 for another case where this needed to be changed.

Styling targeting files-ng action bar menu hosting buttons was not being
applied to the 'cr-button'.

Changes the share-menu button to 'button' element so the correct
files-ng styles for drop-down menu buttons apply.

Bug: 1070604
Change-Id: I6d3da19339ba1fb9ae7ce50cfef88ebc09e785c0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148896Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759530}

ui/file_manager/file_manager/main.html

@@ -358,14 +358,14 @@
                 tabindex="0" hidden
                 aria-label="$i18n{TASKS_BUTTON_LABEL}">
         </button>
-        <cr-button id="share-menu-button" class="icon-button menu-button" tabindex="0" hidden
+        <button id="share-menu-button" class="icon-button menu-button" tabindex="0" hidden
                 menu="#share-menu"
                 aria-label="$i18n{SHARE_BUTTON_TOOLTIP}"
                 aria-activedescendant="share-menu"
                 has-tooltip>
           <files-toggle-ripple></files-toggle-ripple>
           <div class="icon"></div>
-        </cr-button>
+        </button>
         <cr-button id="delete-button" class="icon-button menu-button" tabindex="0" hidden
                 aria-label="$i18n{DELETE_BUTTON_LABEL}"
                 visibleif="full-page"