Add partial magnifier to ash palette.

The partial magnifier will magnify a small portion of the screen, similar to a spyglass.

TEST=./out/Release/ash_unittests --gtest_filter=PartialMagnificationControllerTest.*
TBR=oshima@chromium.org

BUG=616112

Review-Url: https://codereview.chromium.org/2239553002
Cr-Commit-Position: refs/heads/master@{#414124}
 void PaletteTray::UpdateTrayIcon() {
   gfx::VectorIconId icon = palette_tool_manager_->GetActiveTrayIcon(
       palette_tool_manager_->GetActiveTool(ash::PaletteGroup::MODE));
  icon_->SetImage(CreateVectorIcon(icon, kShelfIconSize, kShelfIconColor));
 }

 void PaletteTray::UpdateTrayIcon() {
   gfx::VectorIconId icon = palette_tool_manager_->GetActiveTrayIcon(
       palette_tool_manager_->GetActiveTool(ash::PaletteGroup::MODE));
  icon_->SetImage(CreateVectorIcon(icon, kShelfIconColor));
 }
