First gtest!!! type following command to execute gtest.
./mach gtest "GestureManager*"
Explicitly check all resources required in constructor

Also assign mAnonymousContent explicitly in the constructor.
Give mAnonymousContent a meaningful name as mCaretContentHolder
Delete unneeded null check against mCaretContentHolder

We've assert it in InjectCaretElement.
No need to reset mPresShell
Improve AccessibleCaret::SetPosition()

Convert position to int css pixel should be sufficient.