Add the partial screen magnifier to Chrome OS.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index 78056316518ebd3d7431641b4ad4b6ebe14ea271..1111235a40fbc6ace9aefc283772fe50cb8fd6d4 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -727,6 +727,8 @@ const char kHighContrastEnabled[] = "settings.a11y.high_contrast_enabled";
 const char kScreenMagnifierEnabled[] = "settings.a11y.screen_magnifier";
 // A double pref which determines a zooming scale of the screen magnifier.
 const char kScreenMagnifierScale[] = "settings.a11y.screen_magnifier_scale";
+// A boolean pref which determines whether the partial magnifier is enabled.
+const char kPartialScreenMagnifierEnabled[] = "settings.a11y.partial_magnifier";
 // A boolean pref which determines whether virtual keyboard is enabled.
 // TODO(hashimoto): Remove this pref.
 const char kVirtualKeyboardEnabled[] = "settings.a11y.virtual_keyboard";