Incorrect percentage in Field histogram

[thurka]

Describe the bug
Percentage in field histogram is based on all heap dump instances instead of all instances of particular class. The current % numbers are almost always 0.0%, so it is not very useful.

To Reproduce
Steps to reproduce the behavior:

1. Open heap dump.
2. Select class in 'Objects' view and open 'Fields' detail.
3. Expand one of the Class' fields.
4. Examine % in 'Count' column.

Expected behavior
% in 'Count' field should be based on number of selected class instances.

[thurka]

Fixed for all summarizing views. Field histogram, Merged references and Merged GC roots