Fix invalid nul character in KeyCode accelerator

A recent change in RenderText caused a regression while showing
command accelerators on MAC (see: http://crbug.com/1016676).

The change in RenderText is replacing the embedded control characters
with their equivalent unicode visual pictures. This change should not
cause that regression.

By digging into the code, we found that some strings are incorrectly
providing string with embedded nul character. These controls characters
doesn't have 'glyphs' in font and will triggers expensive font fallbacks
before faling to find the appropriate font.

This CL is removing the leading \0 character from
Accelerator::KeyCodeToMacSymbol(...)

R=robliao@chromium.org,ellyjones@chromium.org
CC=thomasanderson@chromium.org

Bug: 1016676
Change-Id: Iaf3de8e5aec4de4e63e09f1071bf27768d0762d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876468Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Etienne Bergeron <etienneb@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708799}

ui/base/accelerators/accelerator.cc

@@ -231,31 +231,31 @@ base::string16 Accelerator::GetShortcutText() const {
 base::string16 Accelerator::KeyCodeToMacSymbol() const {
   switch (key_code_) {
     case VKEY_CAPITAL:
-      return base::string16({0x21ea, 0});
+      return base::string16({0x21ea});
     case VKEY_RETURN:
-      return base::string16({0x2324, 0});
+      return base::string16({0x2324});
     case VKEY_BACK:
-      return base::string16({0x232b, 0});
+      return base::string16({0x232b});
     case VKEY_ESCAPE:
-      return base::string16({0x238b, 0});
+      return base::string16({0x238b});
     case VKEY_RIGHT:
-      return base::string16({0x2192, 0});
+      return base::string16({0x2192});
     case VKEY_LEFT:
-      return base::string16({0x2190, 0});
+      return base::string16({0x2190});
     case VKEY_UP:
-      return base::string16({0x2191, 0});
+      return base::string16({0x2191});
     case VKEY_DOWN:
-      return base::string16({0x2193, 0});
+      return base::string16({0x2193});
     case VKEY_PRIOR:
-      return base::string16({0x21de, 0});
+      return base::string16({0x21de});
     case VKEY_NEXT:
-      return base::string16({0x21df, 0});
+      return base::string16({0x21df});
     case VKEY_HOME:
-      return base::string16({0x2196, 0});
+      return base::string16({0x2196});
     case VKEY_END:
-      return base::string16({0x2198, 0});
+      return base::string16({0x2198});
     case VKEY_TAB:
-      return base::string16({0x21e5, 0});
+      return base::string16({0x21e5});
     // Mac has a shift-tab icon (0x21e4) but we don't use it.
     // "Space" and some other keys are written out; fall back to KeyCodeToName()
     // for those (and any other unhandled keys).