Fix Clang style-plugin errors in memory_pressure component

Fix Clang style-plugin errors in memory_pressure component

From http://build.chromium.org/p/chromium.fyi/builders/CrWinClang/builds/2796/steps/compile/logs/stdio:

..\..\components/memory_pressure/filtered_memory_pressure_calculator.h(31,3) :
error: [chromium-style] Complex destructor has an inline body.
  ~FilteredMemoryPressureCalculator() {}
  ^
..\..\components/memory_pressure/filtered_memory_pressure_calculator.h(31,39) :
error: [chromium-style] Overriding method must be marked with 'override' or 'final'.
  ~FilteredMemoryPressureCalculator() {}
                                      ^
                                       override
..\..\components/memory_pressure/direct_memory_pressure_calculator.h(41,45) :
error: [chromium-style] Overriding method must be marked with 'override' or 'final'.
  virtual ~DirectMemoryPressureCalculator() {}
                                            ^
                                             override

BUG=82385, 520962
TBR=chrisha

Review URL: https://codereview.chromium.org/1314553003 .

Cr-Commit-Position: refs/heads/master@{#345121}