[Linux] Memory values shown by memory pressure handler logger are not useful

[Linux] Memory values shown by memory pressure handler logger are not useful
https://bugs.webkit.org/show_bug.cgi?id=164589

Reviewed by Michael Catanzaro.

We are currently using the VmSize field from /proc/self/status which is the virtual memory size of the process
and doesn't normally change even when the memory pressure handler manages to release memory. So, most of the
time we see that there's no changes in memory usage in the logs.
We should use the actual memory used by the process, memory that the process can release and then it's relevant
for the memory pressure handler. Using other fields from /proc/self/status we could do something like VmRSS -
(RssFile + RssShme), but there's also /proc/self/statm that provides the same information in a single. The main
different is that statm provides both resident and shared memory directly, but in number of pages, so we need to
multiply by the size of the page.
This patch adds a method to parse /proc/self/statm in its given file, because I plan to use this for the linux
memory sampler that is incorrectly parsing /proc/self/statm.

* platform/Linux.cmake: Add new files to compilation.
* platform/linux/CurrentProcessMemoryStatus.cpp: Added.
(WebCore::systemPageSize): Return the page size.
(WebCore::currentProcessMemoryStatus): Parse /proc/self/statm and fill the given ProcessMemoryStatus.
* platform/linux/CurrentProcessMemoryStatus.h: Added.
* platform/linux/MemoryPressureHandlerLinux.cpp:
(WebCore::MemoryPressureHandler::processMemoryUsage(): Helper function to return the memory used by the process
in bytes.
(WebCore::MemoryPressureHandler::ReliefLogger::platformMemoryUsage): Use processMemoryUsage().

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@208536 268f45cc-cd09-0410-ab3c-d52691b4dbfc