Adds some UMA log entries for input methods.

BUG=367631
TEST=Verified with IME extensions on linux_chromeos.
NOTRY=true

Review URL: https://codereview.chromium.org/606553002

Cr-Commit-Position: refs/heads/master@{#296966}

tools/metrics/histograms/histograms.xml

@@ -9971,6 +9971,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="InputMethod.AutoCorrectLevel" enum="IMECorrectionLevel">
+  <owner>shuchen@chromium.org</owner>
+  <summary>The auto-correction level for suggestion engine.</summary>
+</histogram>
+
 <histogram name="InputMethod.Category" enum="InputMethodCategory">
   <owner>shuchen@chromium.org</owner>
   <summary>
@@ -10005,6 +10010,28 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="InputMethod.VirtualKeyboard.BackspaceCount">
+  <owner>shuchen@chromium.org</owner>
+  <summary>
+    The number of times the backspace key was pressed on the virtual keyboard,
+    while the virtual keyboard was alive. Recorded when the virtual keyboard is
+    closed.
+  </summary>
+</histogram>
+
+<histogram name="InputMethod.VirtualKeyboard.BackspaceOnLayout"
+    enum="IMEVKLayout">
+  <owner>shuchen@chromium.org</owner>
+  <summary>
+    The layout type of the virtual keyboard, recorded when backspace is pressed.
+  </summary>
+</histogram>
+
+<histogram name="InputMethod.VirtualKeyboard.Duration" units="seconds">
+  <owner>shuchen@chromium.org</owner>
+  <summary>How long the virtual keyboard was visible.</summary>
+</histogram>
+
 <histogram name="InputMethod.VirtualKeyboard.InitLatency" units="milliseconds">
   <owner>shuchen@chromium.org</owner>
   <summary>
@@ -44159,6 +44186,12 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </int>
 </enum>
 
+<enum name="IMECorrectionLevel" type="int">
+  <int value="0" label="Off"/>
+  <int value="1" label="Modest"/>
+  <int value="2" label="Aggressive"/>
+</enum>
+
 <enum name="IMEVKLayout" type="int">
   <int value="0" label="Compact"/>
   <int value="1" label="CompactSymbol"/>