Avoid doing malloc in a loop in RenderFlexibleBox::layoutFlexItems

When profiling the silk infinite_scroll benchmark we spend about 4ms in
layout on the flex boxes. Looking at the profile on my mac 2% of that was
inside malloc on a Vector<LayoutUnit>.

This patch adds some stack allocated storage for the Vector and also makes
the Vector be sized and reused so we don't need to malloc it over and
over inside the loop.

Review URL: https://codereview.chromium.org/148763003

git-svn-id: svn://svn.chromium.org/blink/trunk@165929 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Send HeapProfiler.enable only when profiles panel is shown

Otherwise on cross-process navigation we get following console error:

[22766:22766:0127/193003:ERROR:CONSOLE(237)] "Protocol Error: the message HeapProfiler.resetProfiles is for non-existing domain 'HeapProfiler'", source: chrome-devtools://devtools/bundled/InspectorBackend.js (237)

BUG=None
R=alph@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=165858

Review URL: https://codereview.chromium.org/132903010

git-svn-id: svn://svn.chromium.org/blink/trunk@165930 bbb929c8-8fbe-4397-9dbb-9b2b20218538