a11y: Remove extra period in power status accessiblity string.

ChromeVox will read .. literally as two dots. Just use the one period from the .grd file string.

Test: manual
Bug: 747470
Change-Id: Ia5bc2c0a345f1a5b84f4d26bf0141d565f2d1207
Reviewed-on: https://chromium-review.googlesource.com/677567Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504797}

ash/system/power/power_status.cc

@@ -453,7 +453,7 @@ base::string16 PowerStatus::GetAccessibleNameString(
   }
   return battery_time_accessible.empty()
              ? battery_percentage_accessible
-             : battery_percentage_accessible + base::ASCIIToUTF16(". ") +
+             : battery_percentage_accessible + base::ASCIIToUTF16(" ") +
                    battery_time_accessible;
 }