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

Committed: https://chromium.googlesource.com/chromium/src/+/ea3ec1ea7c1ad2ac50f54f50a14b644904f7df02

[hans, 2015-08-24 17:45:02]

hans@chromium.org changed reviewers:
+ chrisha@chromium.org

[hans, 2015-08-24 17:45:07]

The CQ bit was checked by hans@chromium.org

[commit-bot: I haz the power, 2015-08-24 17:45:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1314553003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1314553003/1

[hans, 2015-08-24 18:01:11]

Committed patchset #1 (id:1) manually as
ea3ec1ea7c1ad2ac50f54f50a14b644904f7df02 (presubmit successful).

[chrisha, 2015-08-24 18:29:24]

Thanks for cleaning this up.

lgtm